It seems not that complicated to implement, technically. But, there would probably be a legal/copyright problem: If the user didn’t upload the image to your forum and only provided a link, you don’t have IMHO the right to do that without his consent. You could maybe insert a clause in your TOS for that. I don’t like this idea very much, but you would then probably be OK.
Daniela’s solution works without any consideration like this, but it’s less nice in term of user experience 