AIM supports animating buddy icons in the form of .gif files.

Setting an Animated Buddy Icon

Choose Preferences from the Adium menu. Select the Personal tab. Drag the .gif file from the Finder into the Image Well of the Personal tab. Note: You must drag the file into the image well. The animation of the icon will be lost if you use the Image Picker or press the Choose Icon button to select your buddy icon.

Also, AIM buddy icons must be between 48x48 and 50x50 pixels. When an image is resized, its animation is lost. If Adium has to resize your image down to 50x50 pixels, you will lose its animation. Animating buddy icons therefore can not be more than 50x50 pixels.