Quadtrees never fail to amuse.
Source + binary (C#, Visual Studio 2012): QuadTreeImageToy
Loosely based on http://devmag.org.za/2011/02/23/quadtrees-implementation/
The usual disclaimer applies: batteries not entirely included, unoptimized, it could be done cleaner.. but it was fun to mess with for a few hours. Most of the interesting code can be found in Form1.cs.