This is a good idea. Most spammers who include images use free image hosting services. These companies get an excessive amount of images that are bound to end up in someones inbox as UCE, so it is very difficult for them to police this type of thing. Most of them also provide ways of reporting an image as spam or offensive, although it's normally by clicking the link to the image and then checking a button to mark it as such.
However, most of the time I see images that are simply attached within the email, rather than linked externally.
Not to get off topic, but I think it would be cool if Spamcop could report phone numbers found in email addresses to the phone company they are registered to and have them shut down