This looks like an ideal task for Inscape.
It is free, GPL 2.
It runs on Linux.
It's considerably small, 107 MB download size. That's less than half of Affinity Deisgner, a comparable commercial product. So it should fit on older PCs with little disk space available.
The startup time is certainly not very fast, but once started it's ok. It's a program for creating vector graphics. That is: you can edit the graphics later, whereas in a pixel graphic, your changes are more of a permanent nature. Thus, changing the graphics will be easier.
Regarding simple: well, it has a lot of options, but you needn't use them. You'll find arrows and circles quickly.