This image verification solution is free, I use 16 images in this application.
Each image will always have the same filename and associated code. Images
and their associated code are displayed randomly so this is an ok solution for
free.
If you would like to upgrade, I have an enhanced version ($19.95 donation) which
randomly renames the files so that image1.jpg might have a code of "sailor" on
the first hit. Say you hit the code "sailor" later, the file may be named
image16.jpg - this solution is more secure from hackers, but both work nicely.