I have just installed snow leopard. I'll try to have a look soon.

To Penegal & juanchi, did you download the version I posted ?

Here is a complete working version. I'm not the original developper but he allowed me to fix the script. So I can't post it as a current version in adiumxtras.com.
Get it there : http://www.4shared.com/file/94908967/...cripts.html

I found why it wasn't every time the correct artwork.
Here's a better version: http://www.4shared.com/file/84545779/...rtwork.html

%_Artwork change the icon of every account. It just have to be called once. So, I think place the call in an unused account is the best solution.