Settings - Convert PNG images to JPEG

This option makes ShortPixel Image Optimizer convert all PNG images that don't have transparent pixels to JPEG. This can dramatically reduce the file size, especially if you have camera pictures that are saved in PNG format. In addition, the plugin will also search for references of the image in posts and pages, and replace them. There's three important things to take into account, though:

• The image will not be converted if the resulting JPEG is larger than the original PNG.
• If the original image contains even a single transparent pixel, it will not be converted.
• You can always force the conversion for transparent images by enabling the option below called "Also force the conversion of images with transparency". Please note that this option only shows up if the first one is enabled.
  

To learn more about why images may not be converted to JPG, please check this article: Why aren't the PNGs converted to JPEGs even when I choose this option?