This is a pretty cool tweak to perform on AOL Instant Messenger. You will need Resource Hacker though. Get into ResHacker and open aimres.dll in the AIM95 folder. On the tree open GIFDATA>103>1033. You should see your current sign on graphic. I would suggest backing up this graphic in case you ever want to go change it back. Once you have another graphic edited to go in its place, open the Action menu and click on "Replace other Resourse..." Open your new graphic in the area given... Then copy in this info:

Resourse Type: GIFDATA
Resoures Name: 103
Resourse Language: 1033

Then click "Replace" and Save.

Another cool thing you can do from ResHacker with aimres.dll is edit some of the dialog AIM uses. Open the dialog portion of the tree and costumize some of the messages and screens used in AIM.