Hi and welcome to the forums. The background was removed because it makes aMSN runs slow

. However, you can try to download this
http://amsn.svn.sourceforge.net/viewvc/amsn/trunk/amsn/skins/default.tar.gz?view=tar&pathrev=11459 That is the last version of the Skin before we changed it.
Untar it (with WinRAR in Windows) and go to the aMSN installation folder
- Windows: C:\Program Files\amsn\Scripts\skins\ (or something like that, Mac OS X user, sorry)
- Mac OS X: /Applications/aMSN.app/Content/Resources/Scripts/skins
- Linux: I can't remember it lol
Back up your old "default" skin and copy the new folder you've downloaded.
I hope it can help you =) Good luck!
--- updated link, I put the link for pixmaps before, now the link download the skin
