selling up every think must go

[alanf]

Yes this sort of breach is disgraceful, I agree.
My first husband was a photojournalist and while he worked for the national newspapers they owned his shots and indeed sold them on. His agency stuff is still his copyright and even 15 years after his death I get the odd revenue cheque.
I don't think a few avatars here and there matter though

The most I have ever heard of on a non-professional forum or blog is a "don't use that" message. If you know the owner, an acknowledgement goes a long way. Tags can be added digitally with contact information, but are easily changed. There are ways of embedding information in photos that are harder to remove and can prove ownership, steganography for instance. Most agency sites display a low resolution photo version and supply a full resolution version when you buy or license the image. If you find your image being sold on commercially, chances are it's not the only one they are ripping off. You could try searching for previous complaints and contacting others in the same position.