this. so what if he took it? I would be flattered if I were you. Even if he thinks you didn't make it, he still liked it enough to put it as his.
What if someone stole your social security card? Would you be flattered that someone wants to "be" you?
An extreme comparison, I'm sure, but the fact of the matter is that to some people their avatar is who they are around here, to use their avatar is to masquerade as them in their eyes.
Such a concept might not make sense to someone who changes their avatar as often as night changes to day, but as a person who has, essentially, been using the same avatar for the past 5-6~ years I can tell you that I would be pretty damn annoyed if someone used my avatar and wouldn't change it no matter how much I asked them to