This website page makes me happy. It reminds me of those drawing pads we had as kids, where you lifted a sheet of plastic and your drawing disappears. Only this is in reverse.
What you do: You transform an image into a "magic" Farbzauber picture. Copy and paste the url of your picture into the form which has an explanation in English. Choose outline or black and white, then hit submit. You'll see an outline of your picture like a kid's colouring page. Drag your mouse over the picture to "colour in" and you will reveal it in full colour.
It's part of a Barbapapa fan site which is in German. I wandered around and found a photo colouring section too. It MAY be only good if your browser is Chrome or Safari, I'm not sure. If your kids are Barbapapa fans, you'll find videos, pictures and games about these charming books, like this one which is a scene builder unless my rudimentary Deutsch is letting me down.
This might be fun for kids who are just learning mouse skills as they colour inside the lines. You can't actually go over the lines. It's not very creative but it's fun. It amused me for a couple of minutes! I made sure I chose a piece of Royalty free clip art so that I wasn't breaking copyright. I also used Google Advanced Image search and found several images available that were labelled for re-use.
Move your mouse over my dragon picture below to see the "magic"! I did this via the embed option.
Farbzauber via Barbafan