The usual way I side load books to my Kobo Aura H2O is with Calibre.
I have zip files containing images with different formats, for instance JPEG, GIF, or PNG. The exact formats don't seem to matter. When I try to get Calibre to side load one of these zip files to my Kobo, Calibre will not give me an option to send the file to my ereader, it will instead pop up a dialog that offers to convert the book, but if I tell it to attempt the conversion, it will then fail. Note that Calibre will place zip files containing images that are all of the same format on the Kobo without any problems, and that works for any zip file containing images all of the same format as long as that format is one the Kobo will read.
This isn't the first time I've seen wonky behavior from Calibre on zip files: at one point when I attempted to side load a zip containing images and an extra file (thumbs.db or something named like __MacOSX that apparently Macs add to image directories), Calibre would attempt to interpret the file as an epub and refuse to put it on the Kobo. That also caused it to pop up a conversion dialog that would then fail if I attempted the conversion.
I'm trying to figure out whether this is a limitation in Calibre or a limitation of the Kobo ereader. If I could put a zip containing images of mixed formats without using Calibre, would it work? Is there a way around this problem that doesn't involve needing to convert all the images to the same format first?