AJAX Thumbnail Rebuild is a free WordPress plugin designed to enhance thumbnail management on your site. Unlike traditional plugins that regenerate all thumbnails at once, this plugin rebuilds thumbnails sequentially, which significantly reduces the risk of server timeouts, especially for sites with a large number of images. This is particularly useful when you alter the size of your thumbnails to match a new theme, ensuring that all existing images are updated without overwhelming server resources.

The plugin allows users to select specific thumbnail sizes for rebuilding, providing flexibility and control over the process. Additionally, users can opt to only regenerate post thumbnails, streamlining the task further. However, it's important to keep the browser page open until the entire process is complete to avoid starting over. This plugin requires JavaScript to function effectively, making it a straightforward solution for managing image sizes in WordPress.