Masking

For especially spooky effects, turn your images into greyscale images, and then select other images with the greyscale image as a mask. (Control-Shift-F). Then apply some kind of effect with the mask. If you take something like a photograph, make sure you invert it so it looks more like what you're selecting. (Kind of like a negative...)

One day, I will make a stunning example of this, but that day isn't today.
Original
Lousy QuickCam picture of me rubbing my nose.
Negative/Mask
Result
Plasma, and then Mosaic applied to blank white canvas.