Nice try. However, that is one of the DU provided avatars. If there is a copyright issue, Skinner and co. would have to have paid to use it. Copyright, however, is limited in duration so my guess is that particular drawing is no longer covered.
So, let me get this straight.
You are using the 'I didn't know any better' defense ?

Okay, if you happen to be in the United Kingdom, the image fell into PD in 1985. (life +70 years) HOWEVER - The legality is kinda grey in the United States, because of Clinton's DMCA. -Mainly because a tool was used to reproduce it, (in this case, a PC) and according to the DMCA, the tools aren't legal to use in any way. -It's ridiculous.
But let's continue, shall we ?
Here is a common DU avatar that is still
VERY protected by copyright.

it comes from here -

..Definitely not 70 years old.
The way the DMCA works, is every reception of this image is a violation. So, if you scroll through a page on DU with posters who use this avatar, every time it pops up on your screen is one count. It also gets lodged in your PC's web cache, so you have actually copied it -in some cases, multiple times.
..and the 'I didn't know any better' defense won't cut it in court. You know that.
The odds of anything actually happening to you (or anyone who has just read this post) - pretty much nil -
BUT that doesn't change the fact that a copyright law has been broken, has it ?
And my point is ?
-You seem to be high and mighty with copyrights that you feel have been violated against you, but seem to have no problem doing the same thing against someone else. Get your own house in order first.