3D Screenshot is a GIMP script that can convert any image into a 3D vista style screenshot. 3D Screenshot will only work if the width is at least about 150% of the height of the visible image. Otherwise it will show either an incomplete picture or a blank image.

How to install:
Click on the file and save it to an easy-to-find location (such as desktop). Then move it to \Share\Gimp\x.x\scripts. If GIMP is open, it will need to be restarted before you can use this script.