I thought that Paint.Net was free! Have you been diddled?
If advice seems slow coming, I guess that most of us here are using apps at the more complex end of the spectrum and have little knowledge of apps that are simplified.
I'd do lots of googling about what's out there, read reviews, then take a stab at something - and if it's a paid-for app, download a free trial first - but only download apps from their makers' websites to be safe.
Even then, it's possible that even a genuine app that's malware-free will mess with your computer - if not harmfully as such then in little commandeering ways that are in it's maker's own interests rather than yours. It can be a minefield at the free / cheap end.
But it seems that Photo Studio 5.5 is discontinued, which suggests that it's time to move on?