Any number of questions, usually from first time users, embed images without acknowledging where they come from. Then the moderators manually make a comment prompting them to read the site rules.
Surely the 'upload image' tool should include at least a prominent message about this.
One stage further would be a number of radio buttons which, until one is ticked, disable the upload button. One asserts 'this is my own original work', another 'I have included attribution for this image in my text' (or something similar), another (to address Sam's answer) 'I have modified an image I found [here]'. Maybe there are more options, maybe not, I feel the more complicated it gets, the less useful it becomes.
Moderators have a hard enough job anyway, without having to mandronically do something that the system has a perfectly placed checkpoint to do.
It's similar to, but not a duplicate of, this question from 2013 which talks about Fair Use