I have just completed a migration of a number of sites which use Really Simple Captcha, as part of Contact Form 7 and stand alone.
Some tips for troubleshooting issues (some of this is repetition from other posts) which took me a few minutes to figure out:
1) Really Simple Captcha works great on WP 2.8 series (tested up to 2.8.4 and today on 2.8.5). There have been numerous questions about this in the forums. Running 2.8 series WP is NOT the cause of your Really Simple Captcha plugins not working.
2) Seeing no images? Then:
2.1) Verify that PHP has GD support _and_ Freetype support compiled in. If not you are SOL until you enable this support and need to use a different CAPTCHA solution. Google for "compile PHP GD freetype" for tips on getting this setup working.
2.2) Go to Settings->Miscellaneous and check your upload path. If you have moved hosting providers, moved servers or moved deployment locations this path may not be correct anymore. Really Simple Captcha uses this path to create the images. Which leads to:
2.3) Verify that the web server user has permissions to _read and write_ to the directory wpcf7_captcha directly below your upload path mentioned above. This is where the images are stored.