This program is very good and there is great potential. Here are some items I've noticed during my first test that could make it look more professional and streamlined IMO and are somewhat easy to implement:
+Asterisk in document name signaling there are changes not saved. Also, prevent redundant saving when there are no changes.
+History log. Sometimes you are testing a new brush or whatever and you're not sure when to stop hitting undo to don't accidentally undo some important brush strokes elsewhere in the canvas. Also, the undo/redo buttons should be grayed out when there are no more actions they could go over.
+Brush snap to 45º degrees. Holding Ctrl or V or whatever.
And a bug:
I imported Apterus' brushes and messed a bit with their settings, however, I wanted to restore one of them to their original state. I select it and chose "Load default brush" but it replaced it with a standard circle brush instead and won't allow me to select it anymore. Attempting to "Reset Current Category" pops up the message "Current category is not default. Can not be reset!". So the only thing I could think of was to restart the program, which finally restored Apterus' brushes to the default state.