Shamelessly stolen from my friend Deadzombie over at Neowin...
Create a new image in Photoshop, 128 pixels high by 768 pixels wide. This will give you 6 animation frames 128x128 (768/128 = 6). Make certain the background is transparent.
Set your guides and grids to 128 pixels, 8 subdivisions. Show grid.
You'll need 6 images for each animation section. for a zoomer I start with 48x48 on the left and increase the size each frame to the right.
From left to right the image sizes I use are 48, 64, 72, 96, 112, 128. You can use Gif Movie Gear or iconXP to export .psd files of an icons images sizes.
Using your align tools, align all the images within your selected ares, and save for the web as a 24bit .png with transparency enabled.
Right click on an Object and choose properties. Under the appearance dialog in properties you will see several different states (mouse over, mouse away, show...) most zoomers have only 3 states, although I have made a few zoomers with 16 different animation states before...the point is, you will see the image file for each state...browse to your new .png and load it. Make cetain it is applied to each appearance state.
Also make certain that under the animation tab you have the correct number of frames listed (6). Some people use 4 frames, others 5. I've seen animations with 50 frames before. Yikes!
Just remember to adjust the settings for each animation state or you'll get some strange effects.
Hope this helped.
Visual link here. http://www.neowin.net/bboard/showthread.php?s=&threadid=35499