Minimum role should be read backwards. The user needs an authority number *smaller* than the "minimum" in order to have the permissions apply.
That wasn't the problem. I changed the minimum to "Member", and he is a "Super User". I logged him out and back in, but the red "X" still appears on all images. Any more suggestions?
What policy did you use in the ACL entry? I sounds like the user may not have a necessary permission.
discuss.answer
The problem was in my path. I had "assets/images". It needed to have a final slash at the end... "assets/images/" makes the images show up.
It's a good think to remember that in MODX, all path and URL settings end with a slash.