I have the exact same problem.
Dev of the website in a subfolder called "v2"
Moved from v2 to root folder by first :
- changing urls with advanced edit cforms plugin
- disabling scforms plugin and all plugins
Then Ireactivated and everything worked fine, except the captcha image that is no longer displaying. It was working in subfolder (before move).
I searched everywhere in the database, to find and replace last pieces of code that contained the url with the subfolder but it did not solve.
I have backed up the cforms settings, then when in global settings and chose the "uninstalling / removing cforms" and validated that choice. Then disabled cforms plugin. Finally re-activated it, restored the settings and guess what still does not work for the captcha image.
I hope someone has an idea for this to work as I love this plugin.
Thx