You don't need GIMP for what you want. I use Pinta bc it's easier to work and understand but in this particular case I didn't use that either.
1. Download the desktop image with that magician.
2. Open it in any image editor and crop it, so that the only image left is nothing else but the magician.
3. Upload it to www.tineye.com and wait for the results. Sort them by largest image.
4. The very first image is one with transparent background.