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.
Lousy QuickCam picture of me
rubbing my nose.
Plasma, and then Mosaic applied to blank