Check out
Kindle Comic Creator, Amazon's free tool for creating fixed layout content. you can design for portrait or landscape, different resolutions, import jpg/png/tiff/ppm/pdf and preview for different devices.
I've also seen 'picture' books in mobi format which just embed a series of image files. But while it offers compatibility with old Kindles (Kindle DX, Kindle 2, Kindle 1) it is probably not the way to go in most cases.