[Noughty]

You can actually make picture files into a book. Convert to cbz and use calibre to convert to mobi. It reads like a book, all functions available for a book work for this file too. You can even zoom in a fraction - to have it completely full screen.
The most useful could be screen rotation. If your pictures are landscape you can actually view them in landscape mode.
Words per line actually affect the picture size, unfortunately you can only decrease it.
So from font functions only rotation and words per line actually do something.

It also makes it neater since you can add title, author to the file instead of seeing just a filename in home screen.

Pdf would allow zoom in but it can be too messy if it's Japanese comic of the way to read it. It can be split on the wrong part if it's in landscape mode. Best probably just zoom in in portrait mode.
It would work best if moving on the page was gradual instead of moving to the next part and not showing the previous part since the image gets split at wrong places.