I also encountered a similar issue using Gazebo 5.1.0 on Ubuntu 14.04.
Running gzclient as root corrected the issue for me, so it may be permissions related?
@sean_kugele , was your issue with the Import Image dialog in the Building Editor? A fix for that has been backported to Gazebo5 but there might have been no releases since. @Steven Peters , can you confirm on the release?
If it was another dialog then it might be an unknown issue.