It is also Windows-only (and I'm not going to WINE it when I have GIMP Kolourpaint and Inkscape). I can't afford (or be bothered to pirate) Photoshop (Elements).

Kolourpaint is basically a non-shitty Paintbrush, so probably highly similar to Paint.NET. The only problem I have with it is it uses KDE's open/save window setups, so all my bookmarks in Nemo (GNOME-based) don't show (this may just be a setting or bridging setup I'm missing.)