I had the same problems - very frustrating. I found a web page that had a helpful bit of code on it - I can't remember where I found it, or I would give the link and credit to them - sorry.
Paste this line:
die(captcha_get_font());
Above this line:
function captcha_validate($code) {
In captcha_utils.php, locate in the 'includes' directory of 4images gallery.
For my situation, I found that Frontpage extensions had placed _VTI_CNF folder in both the 'fonts' and 'backgrounds' folders that CAPTCHA uses. CAPTCHA, obviously not that bright, had decided that the _VTI_CNF folder was yet another font, or background, to be used.
The moral of the story:
Delete (safely - make sure that you're not deleting any real data) any subfolders or 'non-font' or 'non-background' files from the two folders that CAPTCHA uses.
~Cheers, Laz.