I've started to implement the user interface. At first I was going to do each image well as a separate control, but it would have been a pain to keep track of all of them. Instead I'm doing it similarly to the fore/background color one, a single control that's divided into many sections. In my case there's 4, one for each size. Then I should be able to simply have another control of the same kind for the mask.
(a bit later)
Looks like the implementation won't be as neat as I wanted it to be. Ideally I'd have an encapsulated control, as a class. But it turns out that won't work since when the refresh/hit test function is called the context is lost (the app doesn't know to which class the control belongs to) so I can't access the private data. I guess I'll have to do it like I did it before, a separate function, which is a friend to the parent editor class, and can access it's data.
I've got it to work like I said above, but it's really messy, I'll have to clean it up a bit tomorrow.