To copy your desktop press the PRINT SCREEN button, this will save the desktop pic on your clipboard. Then past the picture in paint. Then go to the file menu and press SAVE AS.Then upload your picture on photobucket or whatever else you use. Then paste it on here.
Press these buttons at the same time: Command-Shift-3. Because Macs are amazing, it's automatically saved to your desktop, so all you have to do is upload and paste on here!