Surprised it took this long on a forum to get one of these threads... They seem to be pretty typical...
How do you transfer your desktop to a picture that can be posted.
On Windows, minimize everything and do ALT+PRINT SCREEN. Then open Paint and hit CTRL+V. Then all you have to do is save it as a .JPG or .PNG file and find someplace to post it online (like a photo sharing site).
On a Mac, I think you use the Grab application and then run it through Graphic Converter. Same deal after that.