Thanks for taking the time to contribute. Admittedly the current captcha system is primitive, but I was striving for something that didn't rely on the availability of graphic libraries. It's also pretty easy to add more combinations... which I should do, because as you point out 1:16 is insufficient.

If anyone is interested, the images are stored as jpeg images (with no compression) in a mysql blob.

on point #3... excellent point!

When free time and ambition allows I'll see if I can implement these ideas.

Thanks again