I can think of several approaches here. One is to start with a cluster of 16 picture indicators. Put the pictures in them. There are many ways to scale them to thumbnail size. The simplest is probaby the ZoomFactor property of a picture. One of the toolkits - Vision, I think - includes functions to rescale images. There are also VIs posted on this forum to do simple image scaling, and someone recently posted most of the code to do it with .NET. You can then look for a double-click on any of those pictures and determine which one was clicked. I believe you can find example code for detecting a double-click; the basic idea is you check for two mouse-down events within a certain period of time using an event structure.
Another method would be a single picture control displaying multiple images. You would look for two clicks in the same location (or very close to the same location) within a certain period of time, then determine which image is displayed at that location. A good start for this would be Ben's emoticon display VI at http://forums.ni.com/t5/BreakPoint/Starware-A-new-concept/m-p/616094?view=by_date_descending#M4595
