Kamer DINC

Compatible with the most popular plugins. Please report if you faced any problem with any plugin. 
Inserts automatically alt tag/attributes to HTML if alt attributes are empty or somehow missing. 
Works only in single page.
Note: This plugin is written only in PHP. So you can except a better SEO visibility and better crawl statistics.
Credits

WP Auto Alt Image Tag

(PHP Way) This plugin insert to html missing image alt tags/attributes based on each attachment title or filename.

wodey

You know that Site Speed and SEO are the keys to success.
Auto WebP & Alt Optimizer is a lightweight yet powerful tool designed to solve two major problems automatically:
<ol>
<li>Image Size (Speed): It automatically converts your uploaded JPG/PNG images into the modern WebP format. WebP images are significantly smaller (often 30-50% smaller) than standard JPEGs.
<ul>
<li>New in 1.4: We now use the native PHP GD library (<code>imagewebp</code>) to ensure compression settings work on all server environments, bypassing common WordPress cache issues.</li>
</ul>
</li>
<li>Image SEO (Ranking): It automatically cleans up your image filenames (removing hyphens and underscores) and sets them as the Alt Text. Google relies on Alt text to understand images, and this plugin ensures you never forget to fill it in.</li>
</ol>
<h3>Key Features</h3>
<ul>
<li>Native WebP Conversion: Uses low-level GD functions to guarantee file size reduction.</li>
<li>4 Quality Levels: Choose from Low (20), Medium (50), High (80), or Very High (90) quality presets to suit your needs.</li>
<li>Auto Alt Tags: Converts <code>my-product-photo.jpg</code> to “My Product Photo” automatically.</li>
<li>Zero Bloat: No complex configuration. Just install, select a level, and forget.</li>
</ul>
<h3>Why use this plugin?</h3>
<ul>
<li>Boost Core Web Vitals: Smaller images mean faster LCP (Largest Contentful Paint).</li>
<li>Save Storage: Stop storing duplicate versions of images. original files are replaced to save space.</li>
<li>Automate SEO: Save hours of manual work filling in Alt tags.</li>
</ul>
<blockquote>
Note: This plugin processes images upon upload. It does not currently process images already existing in your Media Library.
</blockquote>

Auto WebP & Alt Optimizer

Automatically convert uploaded images to WebP format using native GD library for maximum compatibility, and auto-fill image Alt text for better SEO.

WP Auto Alt Image Tag vs Auto WebP & Alt Optimizer