Hi, and welcome.
As a coder of a graphic program too, I've tested a lot of them on Windows to get ideas.
* Yours looks quite "pro". Nice design, clean one-executable packaging.
* The drawing tools like antialised lines and antialised freehand shapes are quite unusual, very nice !
* Many painting modes which I didn't even know existed. Any documentation ?
* The multi-step Undo is good to have, but you should really make a Re-do to go with it, otherwise people will Undo too much and lose work.
* In the same line of thought, a protection against "quit without saving" may save someone some serious pain.
* The dither pattern that marks the part that you're drawing : I completely understand the question of performance. It is actually very suitable for a lot of tools (preview polygons...) and for many effects where the result is not very visible (when you're applying subtle low-pressure effects). At the same time, it's not the best for "simple" drawing (Normal at 100% opacity), but you can't have the best of both worlds. One remark : The black and white dither texture itself may be annoying because it can display "fuzzy" (unstable) on low-end screens, and on very high-resolution ones it can become totally indistinguishable from grey, which defeats the purpose of a "recognizable" color. I don't have a solution in mind for this.
If I have one suggestion, it's to choose where your editor sits in the middle of the countless free graphic editors for Windows, and focus on its strengths. It's tempting to try do everything at once, but it would take years to accumulate all the features of Photoshop, and this is even without talking of different color spaces (indexed, HSV, 48bit RGB...) or image types (layer groups, animation)
