The problem with these methods is that they pretty much stop any blind user from using these web sites, but that's another issue.
A good way to handle that issue is to also offer a sound byte that reads out the word. This is also hard for automated scripts to bypass, and if a blind person is using a computer you can pretty much guarantee that they have sound.
I don't know of any websites that actually implement both methods though. I guess the issue with implementation is that the graphic image is easy to randomly generate using software, but the sound byte is much more challenging to generate!
Anyway, this is getting off topic, so I will stop here...