I'm now having the same issue. I modified the comprofiler.class.php file so I could send a second email upon registration verification and my captcha stopped working. I reinstalled got it working again then modified my code and once again it stopped working.
Is it a permissions issue when I re-upload the doc?
I just inserted my comprofiler.class.php into the install folder and reinstalled. Captcha still didn't work.
If I don't use captcha the pages work and I get my second email so I know my code is not the problem. Actually if I just modify the file by putting a space in and resaving/uploading I stop getting images. This is very strange.