I like the idea for the caves; I might steal it. Are you doing anything to have water flow through the cave? I'm not sure if stagnant water inside might be a concern. What if a fish or other critter goes in there to die? Do you have a plan?
Something needs to be done about the photos though. Did you try just opening them with MS Paint? Alternatively,
GIMP is available for free and can handle all your image processing needs.
Krylon Fusion is generally accepted as aquarium safe. I'm not sure how well it would stick to PVC though; sanding the inside to roughen it might help. Another idea to try would be smearing silicone around the inside of the pipe and then covering it in sand; I'd try it on a scrap piece of piping first.
Others have used Rit fabric dye on PVC with great success; I'd look into it:
http://www.cichlid-forum.com/phpBB/v...4a816d55558565