Necessary to visit image captcha page, change nothing, and save, before it works

When I install and enable captcha, and enable image captcha on certain forms, visiting those forms produces an unreadable image (normally two bitmap letters will appear) which is impossible to decipher. It is necessary to visit the image captcha admin page and, without doing anything else, click save, for the fonts to be recognized (I think). Then, it works. Take a look at the before and after images included (only modules installed: captcha and admin_menu, d6).