On the cover fo the Powerslave album (the one with the pyramid and pharoah Eddie) on the far right hand side, near the further of the two sphinxes you can see a doodle etched into the wall of a guy peeking over a wall saying "WOT NO GUINESS?"
Maybe Derek Riggs was practicing for the Somewhere In Time cover? Here we go:
1. Bottom left-hand corner of the cover. Behind the foremost lion are the words "Indiana Jones was here".
2. Right next to that, we see what appears to be Mickey Mouse.