There are a lot of neat pictures in the Dig Strategy Guide, some of which are worthy of going up on dig.mixnmojo.com. I apologize if they seem poorly scanned, but it was tricky to align the guide correctly on the scanner bed.
Zaarin, if you're reading this, you're welcome to add these to the site if you like.
Sketches of the five final main characters:
Some color photos of backgrounds:
Note that the steps down to the turtle pool are missing, as seen in a screenshot of an early Dig version. Also, that picture of the top of the central basin suggests that in earlier versions of The Dig, you could climb out of the top of the basin. Weird.