A goodie that I've just done at work. Go up to a colleague's desk when they're away from it and create some folders on their computer desktop titled, for example, 'Ladyboy Porn', 'Big Gay Sucking Studs', etc. Then do a screenshot, save it as a JPEG in Paint and go and delete the folders. Then right-click the desktop, and via properties change the appearance of the desktop, selecting the JPEG you've just created from wherever you saved it to. Then when they return they'll get the shock of their lives and try and delete the offending folders or drag them to the bin and nothing will happen. All other icons on the desktop will still work as the real icons sit directly atop the screenshot ones.
Worked an absolute charm. His face was an absolute picture - and still is.