Why aren't the PNGs converted to JPEGs even when I choose this option?

PNGs won't be converted to JPEGs even if you choose to do so, mainly because of one of these 4 reasons:

1. The resulting JPEG is bigger than the optimized PNG. In this case, the PNG version of the image will be kept. This usually happens with small (up to 50KB) PNG files which are smaller than their JPEG counterparts. 
2. The original PNG contains transparent pixels and you did not choose the "Also force the conversion of images with transparency" option on ShortPixel's settings:

   

3. There are some PHP functions missing from your server installation (like the GD library), which make ShortPixel silently fail to convert the PNG image to JPEG. You can easily check whether this is the case using this test PNG image as it converts well to JPEG if the necessary PHP libraries are present.

4. The original PNGs are big and converting them to JPEGs requires more memory than it is available on your server, so the operation fails with the error "Not enough memory to convert from PNG to JPG". This usually happens on websites hosted on shared hosting plans.
   Sometimes this error can be triggered by another plugin, even if it doesn’t seem related to ShortPixel. In this case, our suggestion is to deactivate all plugins and then try again. If it works, activate your plugins one by one and try the optimization after each activation. This way you will find the plugin that causes that issue.