Depends on what sort of images you need to view, and perhaps on your hacking skills
but there might be some options.
The first thing that comes to mind for me is directfb.org -- you can find some neat information there (all cluttered together in a hard to read fashion imho) but among other things (which I found very interesting) is apparently a way to rebuild Firefox so it doesn't need X (and other things that can be done in DirectFB rather than X). I think this site expects you to be wanting to do some coding, so if there are no ready-to-use apps over there, forgive me.
Anyhow, straight to the point: Have a look here at this fbida project
and Here at fim and fbi
, both of which appear to claim to be ways/means/software for viewing some assortment of images from your framebuffer (without X).
If neither of those appeal to you (or maybe don't work) you may find some other ideas over here at kmandla's blog
which is where I read about the above mentioned tools, but where there is also a LOAD of other cool Linux software and tools for doing all sorts of things, with and without X.
There are PDF viewers available to be sure, that do not require X, but I haven't taken the time to investigate fully, all the potential of working outside X, so YMMV as they say.