Hi Mary-
Thanks for the idea! Not exactly what we were after, but it put me on the right track.
In the end I styled the frontpage like a login page (for non-logged in people), with courses as a sidebar (which is what we were doing initially with the custom theme login page).
However - since we only want to show courses with a certain tag, I had to achieve that with a custom block. Then used custom templating to add the login form in the middle of the frontpage, and only for non-logged in people.
Bit of a long way round, but it had the benefit of working! Can now load the course images and show to non-logged in people.