I'm not sure how, but something happened with the IUSR account on my server. I host OWA and a personal site on the server, and one fateful day, I did something stupid (I'm sure; I don't recall what though) and both sites stared asking for domain logins. OWA also was refusing to show any images on the page.
I did some searching and found out that it's a permission problem with the anonymous login account (IUSR_MACHINE). I changed that account to be an admin account (no other account would work) and it 'works' but I obviously don't want this to be the case.
I followed the instructions here: http://windowsitpro.com/article/articleid/48361/how-can-i-check-the-password-of-the-iusr-and-iwam-local-accounts-on-a-machine.html for resetting the IUSR account password, but after setting the anonymous login to that account in IIS, and restarting it, it produced no change.