The brush tool icon on the toolbar looks like it just disappears if the selected brush has a small radius. It looks like there isn't even a button there!
You can see an example in the attached image. With a brush selected that has a radius of 1, you cant see the icon at all.
The brush stamp icon scales down if the size is below 70 pixels, all the way to nothing. It makes it really hard to see. I think it would be better to simply not do this scaling and leave the brush stamp icon large. There is a number below it that displays the pixel size, so you could still tell how big it is.
When I show the program to friends who are trying to learn it, this is a common point of confusion early on. Often times, they don't even know there is a brush tool icon because its invisible! Removing the scaling is a simple fix that would really help make the UI more approachable and easy to read. Thanks for your consideration!