Rather than creating a single image dynamically (with the GD module or whatever), an easier, more CPU friendly option would be to have separate images for A-Z, a-z and 0-9, and place a selection of them side by side to create a unique word.

Code could be reused from any number of open source counter scripts, and shouldn't require any extra modules.

Just make sure that you use a "*" character or similar in the ALT tags.