2. Use Greg Leuch's easy automated resizing tool on fffff.at or the uploader below:
3. Download the resized image and upload to your Twitter profile!
Advanced Instructions (How It Works)
1. Find your imagery (or use 3fram.es to make an animated GIF and save to your computer).
2.Import your animated GIF into Photoshop (details), or create your animation from still frames.
3. Resize your image to a 48×48 dimension (the secret dimensions!).
4. Save for Web as a GIF, tweak your colors, and save.
5. Upload to your Twitter profile!
More info on FAT Lab.