OpenGG

Simple and small lightbox clone, Modded from Joel Vardy’s lightbox, more info at http://blog.ilooker.tk/
Download and install, that’s all you need to do. And when you post articles with images, this plugin (the js script actually) will automatically lightbox those unlinked images (would not affect those images with links).
For example, I posted an article, with two image unlinked to anything (let’s call them image1 and image2), one image linked to another webpage (called image3). Then when someone click on these three images, image1 and image2 will be lightboxed but image3 will not.
Demos: 
<a href="https://jquery-littlebox-for-wp.googlecode.com/svn/trunk/demo.htm" title="Demo on Googlecode" rel="nofollow ugc">Demo on Googlecode</a> 
<a href="http://blog.ilooker.tk/" title="My blog" rel="nofollow ugc">My blog</a>
<h3>Issue list</h3>
<ul>
<li>Large image overflowing issue;</li>
<li>Iframes and flash objects unsupported yet;</li>
<li>Image groups unsupported yet (one click one image, no prev/next);</li>
<li>Not so customizable for users yet;</li>
<li>UI design improving needed.</li>
</ul>

jQuery Littlebox for WP

Simple and small lightbox clone, Modded from Joel Vardy's lightbox, more info at http://blog.ilooker.tk/

WordPress.org

This plugin optimizes the loading of images (and videos) with prioritization to improve <a href="https://web.dev/articles/lcp" rel="nofollow ugc">Largest Contentful Paint</a> (LCP), lazy loading, and more accurate image size selection.
The current optimizations include:
<ol>
<li>Add breakpoint-specific <code>fetchpriority=high</code> preload links (both as <code>LINK[rel=preload]</code> elements and <code>Link</code> response headers) for image URLs of LCP elements:
<ol>
<li>An <code>IMG</code> element, including the <code>srcset</code>/<code>sizes</code> attributes supplied as <code>imagesrcset</code>/<code>imagesizes</code> on the <code>LINK</code>.</li>
<li>The first <code>SOURCE</code> element with a <code>type</code> attribute in a <code>PICTURE</code> element. (Art-directed <code>PICTURE</code> elements using media queries are not supported.)</li>
<li>An element with a CSS <code>background-image</code> inline <code>style</code> attribute.</li>
<li>An element with a CSS <code>background-image</code> applied with a stylesheet (when the image is from an allowed origin).</li>
<li>A <code>VIDEO</code> element’s <code>poster</code> image.</li>
</ol>
</li>
<li>Ensure <code>fetchpriority=high</code> is only added to an <code>IMG</code> when it is the LCP element across all responsive breakpoints.</li>
<li>Add <code>fetchpriority=low</code> to <code>IMG</code> tags which appear in the initial viewport but are not visible, such as when they are subsequent carousel slides.</li>
<li>Lazy loading:
<ol>
<li>Apply lazy loading to <code>IMG</code> tags based on whether they appear in any breakpoint’s initial viewport.</li>
<li>Implement lazy loading of CSS background images added via inline <code>style</code> attributes.</li>
<li>Lazy-load <code>VIDEO</code> tags by setting the appropriate attributes based on whether they appear in the initial viewport. If a <code>VIDEO</code> is the LCP element, it gets <code>preload=auto</code>; if it is in an initial viewport, the <code>preload=metadata</code> default is left; if it is not in an initial viewport, it gets <code>preload=none</code>. Lazy-loaded videos also get initial <code>preload</code>, <code>autoplay</code>, and <code>poster</code> attributes restored when the <code>VIDEO</code> is going to enter the viewport.</li>
</ol>
</li>
<li>Responsive image sizes:
<ol>
<li>Ensure <a href="https://make.wordpress.org/core/2024/10/18/auto-sizes-for-lazy-loaded-images-in-wordpress-6-7/" rel="nofollow ugc"><code>sizes=auto</code></a> is set on <code>IMG</code> tags after setting correct lazy-loading (above).</li>
<li>~~Compute the <code>sizes</code> attribute using the widths of an image collected from URL Metrics for each breakpoint (when not lazy-loaded since then handled by <code>sizes=auto</code>).~~ (This has been removed due to an <a href="https://github.com/WordPress/performance/issues/2098" rel="nofollow ugc">issue</a>; use <a href="https://wordpress.org/plugins/auto-sizes/" rel="ugc">Enhanced Responsive Images instead</a>.)</li>
</ol>
</li>
<li>Reduce the size of the <code>poster</code> image of a <code>VIDEO</code> from full size to the size appropriate for the maximum width of the video (on desktop).</li>
</ol>
This plugin requires the <a href="https://wordpress.org/plugins/optimization-detective/" rel="ugc">Optimization Detective</a> plugin as a dependency. Please refer to that plugin for additional background on how this plugin works as well as additional developer options.
👉 Note: This plugin optimizes pages for actual visitors, and it depends on visitors to optimize pages. As such, you won’t see optimizations applied immediately after activating the plugin. Please wait for URL Metrics to be gathered for both mobile and desktop visits. And since administrator users are not normal visitors typically, optimizations are not applied for admins by default.
Your site must have the REST API accessible to unauthenticated frontend visitors since this is how metrics are collected about how a page should be optimized. There are currently no settings and no user interface for this plugin since it is designed to work without any configuration.

Image Prioritizer

Prioritizes the loading of images and videos based on how they appear to actual visitors: adds fetchpriority, preloads, lazy-loads, and sets sizes.

jQuery Littlebox for WP vs Image Prioritizer