If you don't want an animated icon, just use the first frame, but if you do, save each frame by right-clicking the costume, then go here http://gifmaker.me/ and upload them to make them into a gif! (Assuming I win, which I very well may not xD)
I just copy pasted the instructions from @Dragonlover09 cuz lazieness, and I'm pretty sure she won't care. She's a lot better at explanations too so