Thanks for all the good suggestions. I tried both NeuroLens (http://www.nitrc.org/projects/nldo/) and MANGO (http://ric.uthscsa.edu/mango/ ). Both downloaded, installed and opened my image very simply on the MAC. No manuals, no configuration, very easy.

I do very much like FSLview (http://www.nitrc.org/projects/fsltest/), but the installation requires running an install script; it works well, but doesn't quite fit the 'simplest' objective of this particular discussion thread.