Yep, me too. I think I logged this in Bugzilla.
I fixed this temporarily on the box I needed it working for, but the updates kept overwriting the theme directory. Probably should make it read only until the problem is fixed.
There is no images directory for the mailonly theme, so you need to create it. Two of the images you need (webmail.jpg, passwd.jpg) are in /usr/local/cpanel/base/frontend. The others I just copied over from the iconic or default themes. As I said, the little patch I made worked fine until the next update.
