I think the only thing I miss from Microsoft Windows, it is “paint”. I can’t find anything in Linux that is simple and reliable. I have used pinta, gimp and others but they break or they are too pro.
So, if you have internet connection. This is your friend
And if you want something a bit more pro like Visio, then this one
I recommend both a lot!
I know you have GIMP, but I just a simple tool to draw red squares!