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 these 4 reasons:
1) the resulting JPEG is bigger than the optimized PNG. In that case the PNG version of the image will be kept. Please note that this happens usually with small (up to 50KB) PNG files which are smaller than their JPEG versions.
2) the original PNG contains even a single transparent pixel and you did not choose the "Also force the conversion" option in ShortPixel Settings:
3) there are some PHP functions missing from your server installation (GD library is not installed) and then ShortPixel will silently fail to convert a 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, 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.