@FaceDeer,
It's already in there. By default, it's only used for HTML format, but you can use for specific sites or for epubs, too.
Code:
## This switch prevents FFDL from doing any processing on the images.
## Usually they would be converted to jpg, resized and optionally made
## grayscale.
no_image_processing: true
However, I've also made note that I should look and see if I can make the png to jpg conversion handle transparency better.