How to Thumbnail a Wallpaper

I've been watching for a while now, but this is my first post here, so bear with me.

Can anyone tell my how to thumbnail a wallpaper so that it is is easier to post it. I've noticed that most thumbnailed wallpapers have been linked back to Image Shack, but I use Photobucket and I was hoping that I wouldn't have to get a whole new account to be able to use that technique. Any thoughts?

EDIT: With some help, I figured it out! Thank you SO much!