In my mind, a forum is more useful when people can read through a guide with pictures without having to click on each image to display it individually. But I guess not everyone can look past the fractions-of-a-second lost when scrolling past an off-topic picture. 
Agreed. The cookie stuffing exploit is real enough, though. It would be easy enough to add some Javascript to detect whether or not the resource loaded in the img src url is actually an image, and if not, then all users who have Javascript enabled would get a huge warning (and mods could easily see them and delete).
eg:
http://stackoverflow.com/questions/3499941/javascript-check-if-img-src-is-valid
http://stackoverflow.com/questions/3744266/how-can-i-test-if-a-url-is-a-valid-image-in-javascript