If you are thinking on where the images came from, the easiest (from a license standpoint) is usually to make your own.
You of course does not have to make your own if you can find images that are released under a
license that are compatible with GPL v2. (Note that except for CC0 most of the creative commons licenses are
not. Probably due to clauses that limits reuse under certain circumstances.)
One way around it is of course to ask the copyright holder/photographer/illustrator for permission to use it for work that will be published under GPL v2. It have been done before.
If you are reusing images from somewhere else, please attribute them. This will help if someone later claims they are stolen (in the past it have happened with 3D models here, however they was most certainly stolen).
If you are thinking image file formats png is the most common.