create some interesting images, or download some
start with one layer, activate the slideshow module
tweak the colors, contrast, etc
duplicate this layer, change the range and timing of the slideshow on the new layer
tweak the colors, contrast etc on the new layer
repeat if desired
try different keying on each layer, rearrange them, etc
example:
http://flickr.com/photo_zoom.gne?id=336843078&size=o