Hi! I'm experiencing the same issue.
On the website with WordPress 3.5.1 and FSCF 18.104.22.168, the captcha images fail to appear due to "Cannot modify header information" warnings.
However, on the other website that's still on WordPress 3.5 with FSCF 22.214.171.124, the captcha images are fine. Both sites use the same theme and plugins.
Another possible factor is that my production server runs php 5.2 with safe_mode enabled (not something I have control over, unfortunately).
I disabled captcha for now but would prefer having them.
Any ideas what I could do to make captcha work?