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Very useful WordPress plugin to resize images before they are being uploaded. Just drag and drop big images and don’t worry about your bandwidth or server space usage.
Everything is configured in “Settings” > “Media”.
It is using power of modern browsers to do the job – Chrome and Firefox would use HTML5 method and other browsers – Flash. 
Generally it looks like Chrome is the best for the job (<a href="http://www.plupload.com/" title="plupload.com" rel="nofollow ugc">plupload.com</a>)

Browser Resize Images

Kraken.io

⚡ Compress, resize and convert your WordPress images to WebP, AVIF and more — automatically. Faster pages, smaller files, same quality.
Slow pages cost you visitors, search rankings and sales — and over-sized images are almost always the culprit. Kraken.io Image Optimizer fixes that on autopilot: it compresses, resizes and converts every image you upload — and every thumbnail WordPress generates — so your whole site loads lighter and faster, without you lifting a finger.
Every upload is sent to <a href="https://kraken.io" title="Kraken.io Image Optimizer" rel="nofollow ugc">Kraken.io</a>, one of the most advanced image-optimization APIs on the web, optimized in the cloud, and pulled straight back into your Media Library. No binaries, no command line, no server tweaks — install it, add your API key, and stop worrying about heavy images.
At its heart is Kraken.io’s intelligent lossy engine: for every single image it finds the exact point where the file is as small as possible while staying visually indistinguishable from the original. Expect to cut 60% or more off your image weight — exactly the win Google PageSpeed and Core Web Vitals reward.
From a personal blog to a busy WooCommerce store with tens of thousands of products, it’s an install-and-forget solution that just works.
<h4>✨ Why Kraken.io</h4>
<ul>
<li>Intelligent lossy & lossless — pick maximum savings with no visible quality loss, or pixel-perfect lossless. Per-image JPEG quality and chroma subsampling control for power users.</li>
<li>Convert between formats — turn uploads into WebP, AVIF, JPEG, PNG or GIF. Reach for next-gen WebP or AVIF for the biggest savings (and to clear Google’s “Serve images in next-gen formats” audit), or switch between the classics — whatever fits your site.</li>
<li>Optimizes everything WordPress serves — the full-size image and every thumbnail size (thumbnail, medium, large, …) that actually reaches your visitors through responsive <code>srcset</code>.</li>
<li>PDFs too, not just images — PDF files in your Media Library are compressed through the same Kraken.io engine, so documents download and embed faster.</li>
<li>Smart resizing — cap oversized uploads to a maximum width/height; resized images are enhanced to stay sharp and avoid haloing.</li>
<li>Bulk “Krak ’em all” — optimize your entire existing library from a dedicated Bulk Optimize screen or the Media Library bulk action, with live progress.</li>
<li>A control panel that tells you everything — a Kraken.io summary panel on the Media Library, Add Media and Plugins screens (plus a Dashboard widget) shows your connection status, plan, quota usage, active settings and supported formats at a glance.</li>
<li>Per-image savings & error badges — see exactly how much each image saved, right in the Media grid and list.</li>
<li>Plays nice with your stack — works with multisite, page builders, CDNs and local/dev sites. Detects other optimizer plugins and offers one-click deactivation so they don’t fight over your images.</li>
</ul>
<h4>🖼️ Supported formats</h4>
JPEG, PNG, animated GIF, WebP, AVIF, HEIC/HEIF and PDF. (For security, raw SVG upload is intentionally not enabled — see the FAQ.)
<h4>⚡ Built for Core Web Vitals</h4>
Heavy images are the number-one cause of slow pages. By compressing every size, converting to WebP/AVIF and right-sizing oversized uploads, Kraken.io directly targets the audits that move your score: Properly size images, Efficiently encode images, Serve images in next-gen formats and Largest Contentful Paint.
<h4>🎛️ Your images, your control</h4>
<ul>
<li>Choose intelligent lossy or lossless, globally.</li>
<li>Convert uploads to WebP/AVIF/JPEG/PNG/GIF on the fly — synced live between the panel and settings.</li>
<li>Preserve selected EXIF tags (Date, Copyright, Geotag, Orientation, Profile) or strip them for the smallest files.</li>
<li>Auto-orient photos from phones and cameras by their EXIF orientation.</li>
<li>Decide who can optimize — all logged-in users, authors and up, or administrators only.</li>
<li>Pick exactly which image sizes to optimize (the large 1536×1536 and 2048×2048 retina sizes are off by default to save quota — turn them on if your theme serves them).</li>
<li>Optimize on upload automatically, or defer and bulk-optimize later.</li>
</ul>
<h4>🔒 Private & secure by design</h4>
Your API credentials are stored write-only and masked — they are never pre-filled into the page or exposed in your site’s source, so other users on the dashboard can’t read them. Every action is protected by nonces and capability checks, uploads are content-verified, and all optimization happens server-to-server with Kraken.io. No secrets ever reach the browser.
<h4>🚀 Get started for free</h4>
<blockquote>
Create a <a href="https://kraken.io/pricing" title="Kraken.io – Plans and Pricing" rel="nofollow ugc">free Kraken.io account</a> — no credit card required — and get testing quota to try the plugin and the rest of the Kraken.io toolset:
<ul>
<li>A fully-featured optimization API with official libraries for PHP, Node.js, Python, Ruby, Java, Go and .NET</li>
<li>Web Interface (free) and Web Interface PRO with resizing and sync-to-Dropbox</li>
<li>URL Paster and Page Cruncher to optimize images in bulk from anywhere on the web</li>
<li>Optimization history and stats, and Kraken.io Cloud Storage</li>
</ul>
</blockquote>
You can use a single API key across as many sites as you like — there is no per-site license.
<blockquote>
★★★★★ Excellent option for image optimization 
 “The real power of Kraken is their ‘intelligent lossy’ optimization. I use it on all my sites and have never once needed to roll back an image because of too much quality degradation. It is a perfect solution as is.” — <a href="https://profiles.wordpress.org/collinmbarrett" rel="nofollow ugc">collin</a>
★★★★★ Quality results, quality service 
 “The plugin works really well and effortlessly, and the support is prompt, thoughtful, and thorough. I’m hooked.” — <a href="https://profiles.wordpress.org/illustrata" rel="nofollow ugc">illustrata</a>
★★★★★ Optimize according to Google PageSpeed 
 “Kraken was instrumental in optimizing images to comply with Google’s PageSpeed analyzing tool. Our travel blog now sports Google’s ‘mobile friendly’ tag for mobile searches.” — <a href="https://profiles.wordpress.org/qualterio" rel="nofollow ugc">Walter Schaerer</a>
★★★★★ Perfect solution to speed up site! 
 “I love this plugin! All my questions are quickly responded to and I see a huge saving in image size without losing quality. Highly recommend!” — <a href="https://profiles.wordpress.org/ezone69" rel="nofollow ugc">ezone69</a>
</blockquote>
<h4>Connect with Kraken.io</h4>
<ul>
<li>Website: <a href="https://kraken.io" rel="nofollow ugc">https://kraken.io</a></li>
<li>Plans & pricing: <a href="https://kraken.io/pricing" rel="nofollow ugc">https://kraken.io/pricing</a></li>
<li><a href="https://twitter.com/KrakenIO" title="@KrakenIO" rel="nofollow ugc">Twitter / X</a> · <a href="https://www.facebook.com/krakenio" title="Kraken.io" rel="nofollow ugc">Facebook</a> · <a href="https://github.com/kraken-io" title="Kraken.io on GitHub" rel="nofollow ugc">GitHub</a></li>
</ul>

Kraken.io Image Optimizer – Compress, Convert to WebP & AVIF, Resize & Bulk Optimize

- It works with 1 image but not at Google PageSpeed standard
- It doesn't work with bulk action
- The worst thing is I inputted my credit card there and they refused to let me cancel my subscription, what a very bad thing from kraken.io

Does not work for bulk actions, subscription cannot be cancelled

As others are noting here, their customer service is terrible. In fact I'd use the word "non-existent".

After signing up with an account I decided that the service was no longer what I needed and contacted them to delete my account. Multiple times, in different ways, I've contacted their customer support about this and never had a response. Currently breaching GDPR rules, they're going to be reported.

Thank goodness I never handed over any payments to them.

Terrible Customer Service

Optimize images automatically — compress, resize and convert to WebP, AVIF & more with Kraken.io. Faster pages, smaller files, same quality.

Browser Resize Images vs Kraken.io Image Optimizer – Compress, Convert to WebP & AVIF, Resize & Bulk Optimize