Separating photos from other images algorithmically?
Difficulty depends on what you really want to classify, i.e. what exactly your data base of images is made ok.
For simple hand-drawn or computer art, I would think of just looking the color histogram (but for complex art with gradients; but still these gradients are likely to induce a very smooth colormap).

At the other extreme, distinguishing realistic computer graphics image from reality is probably impossible.