If you're thinking of going on a coding spree soon, I've got a minor feature idea that you might like to slip in. Would it be possible to have a setting to tell FFDL not to convert or recompress images into jpeg and instead just copy them as-is? I encountered a png diagram near the end of the last chapter of Flight of the Alicorn
that got turned into a solid black square by FFDL, presumably because transparency was used in the original png to produce the not-black parts of the diagram and the jpeg converter just stripped off the alpha channel.
Also I just get a little bit twitchy about lossy compression in general and would prefer to have "archive-quality" copies of fanfics stashed away, regardless of the filesize or the strain large images may put on lower-end ebook readers. But since it's just me being twitchy I wasn't going to bother you until I hit this more practical example of compression causing problems.