Rough image loading.

- png image loading from file. press the "save" icon to do it.
- discovery: the app lags a lot when handling mildly large images. At
100x100 px on my computer (Pentium dual-core processor, 4GB ram), it
takes about a second to load, and takes a long time to zoom and scroll.
Drawing is not too bad though. Possible solutions: put an arbitrary cap
on max image dimensions, or maybe there is a better way to re-draw the
rectangles on screen.