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Everything mentioned so far is probably fine.

I use my own tool which you can get here (if you want a tutorial I can write one for you): https://fanzyflani.itch.io/pixra

It only outputs 256-colour tga files, so you'll probably want to convert them for use in your engine.

I also use GIMP from time to time. It can be used to turn those tga files into other formats.

But basically, if anything looks bad, make it look less bad, and repeat this process until it's good enough or you've had enough. Tutorials will help you make things look less bad, especially that one neptuneisadrift linked.