Hmmm this is actually a very nice problem. I can think of quite a few approaches but they all require playing with some graphical modules (GD perhaps?)

If i we're you, i'd roll my own solution. Even if that's easier to crack, the rewards are smaller than that of a system that everybody uses.

The most basic approach would be to make an image of characters on a "grainy" background. You might even make the characters grainy too, so only the density of dots defines a letter.