Hi there.
I'm having the same problem. Some web JPGs seems to be infected, some attachments seems to be infected and even some ".js" (javascript) files seems to be infected.
I don't believe the same virus can infect such different types of files, so I'm guessing it's the antivirus.
The fun about it is that I have 4 computers in my home running the same Avast! Home Edition, with the same virus database, and this problem is only happening in one of these.
Hope to get an answer soon, as I had to disable my antivirus for now.