Ah, thanks. I'm using GIMP, and the only way to get shapes there seems by manually creating them. For this use case, I might just go to Paint from now on. Apparently much quicker that way.

If you're using Windows, there's been a small application called Snipping Tool included in all Windows versions (consumer editions, anyway) at least as far back as Win 8. It allows taking screenshots (it's most useful for partial-screen captures, hence the name) and doing very simple editing, like adding simple colored boxes around specific elements of the picture. I'm unfortunately still on Win 8, though, so I don't know how the later versions work or what features are included.

JFI: The snipping tool pretty much remained unchanged until Windows 10. In Windows 11 it was replaced with a new tool that also supports the design language of Windows 11. It has a few new functionalities (like a ruler and the option to cut out a part of the image, new options for markers), but remained pretty much the same.

It can still be started through the start menu typing "snip" or it can be started using the overlay method by pressing Win+Shift+S.

One of the most useful tools ever, I used it daily for work.

Didn't know about this one, thanks. It's available on Windows 7, which I'm still on. Handy, but only freehand drawing, no shapes sadly. Nevertheless, good to have.

I use a small and free program called Paint.net. It's kinda MSPaint but a bit more powerful but still lightweight and open is seconds.
It's perfect for GOG prints because if you save as jpeg, it will allow to reduce quality to fit the 400kB.

I use a old version (from portableapps) I have saved but let-me check if I find a link.

Edit: link to the website https://www.getpaint.net
Portable apps

Note that portableapps website is bundling some crap with the programs but you still can get the desired program only if you 7unzip the "installer".

Thanks a bunch, looks interesting.