I know GIMP exists, but it's not worth learning to use it. I have no
artistic talent, so a very crude program is just fine for making crappy
pictures.
For those not familiar, mspaint is a really rudimentary image editor for
windows. The pictures you can make with it will generally look terrible, but
there's almost no learning curve, and for me the barbaric quality is kind of
endearing. For example, here's something I made in mspaint over two years
ago, the kind of thing I'm looking to do:
https://gobblin.se/u/christian/m/math-categories/
