One issue I'm having with designing the user interface is that testing it on-device and using a standard photo editor take too long. My solution: Another custom tool!
This one's called UIDesigner, and does exactly what it says on the tin - creates minimalistic user interfaces in seconds.
Currently, it's quite limited. You can't "snap" objects in place, so you just have to guess. You can't save your work unless you take a screenshot. On Linux, this isn't a huge issue since our screenshot tools are actually good, unlike SOME operating systems. Luckily for users of those, I'll be adding a proper export feature soon. It can also only do text, which is *almost* everything you need. I'm also going to be adding in lines, for designing menus.
It's very easy to use. Click which text size you want, type in what you want it to say, and click where it goes. Made a mistake? Click the X in the lower left and then click the text object you want to delete.
Of course, this is also FOSS - you can download it here.