image not available when site is offline for maintenance

I originally posted a message about this as a Captcha module issue, but didn't get any response. I now think this is more related to the textimage module -- if I just use the math verification, it works fine.

I use image verification when logging in, and all groups have access to the textimage module.

If the site is offline for maintenance, the captcha image is "not available" on the login page. This makes it impossible to log in.

If the site is online, the image appears. Also, if I'm logged in and turn the site off for maintenance, I still get captcha images elsewhere on the site. However, if I open a different browser and try to log in, the image is again "not available."

Therefore:

Site offline: no captcha images for anonymous users
Site online: captcha images available for anonymous users

Maybe it's just a permissions thing, but I can't figure it out. Anyone else see this?

Brad