I would disagree that the majority of images on the web are photos. Yes, there are many photos on the web, but I'd wager the vast majority of images (or at least the vast majority of image requests) are tiny bits of connective tissue of web layout - little rounded corners, gradient backgrounds, icons, etc.
Rounded corners and gradients can both be handled without images these days, but even if you use images for things like that, that bandwidth (certainly) and number (probably) of photos dwarfs them.
