For quite a while I had the X-Files "I want to believe" poster with my les paul crudely photoshopped over the UFO. That was because of a thread I started when I was new here - I claimed to be the new owner of Beano complete with tale about a mysterious uncle, a meeting in a pub and a visit from Ringo Starr. It went well, considering.
My current one is because of a now deleted thread - the OP put up some mediocre pics of his guitar with a big tale about being a pro photographer and how no-one was allowed to use his pics because COPYRIGHT. Sharks sensed the blood in the water, there was a short feeding frenzy and the OP and thread were gone forever. I had taken one of his shots, and 'shopped it (badly, again...) onto a mug to use somewhere in the thread.
After he was gone, I used the pic as avatar because I wanted a change. A mod spotted it, pulled it and warned me against breaching copyright so I took a pic of my guitar at the same angle and put that on the mug with the added copyright message.
I'm getting sick of it now though, I'll probably take a photo of the cat and use that instead.