[kovidgoyal]

I think it only happens for for some .png files -- the tool calibre uses, optipng creates them. However, it is supposed to only create them when the -backup or -keep options are provided, which calibre does not do. So it looks like a bug in some versions of optipng. I can add a check to calibre to look for and delete these files after optipng runs. They are harmless, being zero bytes in size, (apart from causing epubcheck to complain) so you can just delete them from the epub files, or leave them in there -- it does not matter.