Pegasaas Accelerator WP contains methods which can automatically size your images.  There may be situations, however, where the plugin cannot optimize in-page images such as those that are used as background images, or those images that are injected into the HTML via JavaScript by a theme or plugin.  This typically happens for the mobile PageSpeed score for images that are above-the-fold., as most images are sized for desktop browser resolutions.

If you are still getting this warning, you can identify the image(s) in question and properly size them with the WordPress Media library cropping tools.

A warning if you are going to resize an image through the media gallery: be careful as this may negatively impact the desktop view of your pages.  You may choose to ignore this warning as the potential savings may only be a few hundred milliseconds of overall page load time.