Thought I'd provide a visual update..
The fileselector is currently broken whilst I'm converting the GUI to use the external data, and I haven't yet converted the palette options. Most of the other GUI panels are now in.
I've added 9 preset brush options which use an external data file for the size and shape values (so they can be changed externally). I tried icons at 24 x 24 with 8 x 8 spacing, and at 16 x 16, but the first looked too big and the last was too small for my liking. So I'm using 24 x 24 icons with no spacing which seems about the right balance to me, and still allows me to provide a visual guide to the various small brush presets (including the pixel arrangements for each brush). You can alter the shape and size of the brush if you want larger sizes..
Speaking of which, I've rewrote the slider code and they now have proper left and right working arrow buttons (haven't done the vertical sliders yet).
The GUI panels can now be changed in size, colour and position (so you could have the brushes at the bottom for example) by altering the external files.
These are still things to do, but most of that is now done.
I plan to have GUI space for layers, and a workspace (for custom brushes and the various images to be edited) at the bottom of the GUI panel. Larger window sizes will allow more gui space for some of the GUI panels as well.