That just defies all logic.
In "real life", I've never seen a raised footprint.
If I stare at the top image (tan), then flip to the bottom image (grey), then the footprints in the grey image initially appear depressed (normal). Then they will flip to look raised.
Looking at the thumbnail, the footprints all look raised.
Part of the illusion is that the footprints are turning out to be lighter in color than the surrounding medium.
The sand is also very non-distinct, so it isn't as if you can see blades of grass and compare the footprint to the blades of grass.