It wouldn't provide any meaningful security; think of how difficult it is to make a CAPTCHA that is both readable for humans and hard to beat for computers. Adding security to a conversation is a solved problem in the technical sense (use encryption such as GPG), the only reason that it's not in widespread use is that people can't be bothered.

Writing text onto an image is easy. Graphical hit counters are almost as old as the web. The first ones just lined up images with preprinted numbers on them, but as soon as CGI became popular, people had libraries to write the text on top of an image too.