Where are the "user backdrops" in Photo Booth?

I am supposed to be able to add a custom image backdrop to Photo Booth by going to the last effects page and dragging an image file into "any effect labelled User Backdrop." However, I don't see any effects labelled "User Backdrop" and dragging image files into another "effects" panel does nothing.

Photo Booth shows me "Normal" and 24 other available effects. First page: space alien, nose twirl, chipmunk, lovestruck, dizzy, blockhead, bug out, frog; second, sepia, black and white, plastic camera, comic book, color pencil, glow, thermal camera, X-ray; third and last page, bulge, dent, twirl, squeeze, mirror, light tunnel, fish eye. No "user backdrops."