Desktop Wallpapers
For Windows XP/Vista:
Choose your screen size and a full-size wallpaper will open, right-click on it and select "Set As Background".
For Mac OS X:
Choose your screen size and a full-size wallpaper will open, right-click (or hold control and click on it) and select "Save Image to the Desktop". Drag this newly added file onto your current wallpaper seen in the Desktop & Screensaver section of the System Preferences utility found under the Apple menu.
Buddy Icons
For Windows XP/Vista:
Click on the Buddy Icon you'd like to use and it will be automatically added to your AIM, AOL instant messenger
For Mac OS X:
Drag the Buddy Icon you'd like on top of your current icon in the iChat Buddy List window.