First off thanks for your donation! Unfortunately, I may have to disappoint since this program is only a level editor, not an image editor. For editing tile graphics I'd recommend using a dedicated image editor like GIMP or Photoshop. You can use Tiled to preview and try out your tiles as you paint them, since it generally reloads the image automatically when it changes.
I'm happy to support projects like this!
It would be great if you could consider some simple pixel paint functionality at some point. I find that theres a lot of back-and-forth into the tileset when creating a map, so it would be nice to just do minor tweaks without opening another package. Also, at the moment I am upgrading the tileset of a vintage game, the tiles are a mess and its almost impossible to understand them without seeing the changes in the map, and seeing which tiles goes where in realtime.
I know that doing a pixel-editor can open a can of worms in terms of functionality, but some basic pixel placing would be enough I think :)