1. Find an image you would like to use and save it to your computer.
2. Go to www.tinypic.com
and upload the picture. The site will give you an "IMG URL" for your picture.
3. Go to the "User CP" at the top left of the page.
4. Go to "Edit Avatar" on the left side.
5. Under "Use Custom Avatar," enter the URL.
Now, I havent tried directly uploading from my computer to ISH but you can try it to see if it works. I can't guarantee that method, but the one above is how I got mine.