I have categories of images (Target Books, Missing Adventures, Misc Fanzines, That drawer of random stuff in my filing cabinet, various ranges of DWM and so on) and they get selected at random, once I've got a category, however, I then pick an image partly because I didn't have the patience to make a list of every single Doctor Who image I have, and partly because its nice if I feel I actually have something to say about the image, or its a bit topical or some such.

So they're more semi-random.