Writing an editor from scratch is a lot of work. That's one reason why I would use an existing editor whenever possible. For example, TrenchBroom (Quake editor) + func_godot seems to be getting more popular among Godot users, and Tiled looks pretty good for 2D games. For game data management I've seen CastleDB (never used it though), which now seems to be integrated into Hide, a full-blown 3D editor.
Either way, once you get the tooling up and running, the next big step is actually designing a game and creating all the content. :)
One thing I particularly like about Krita is its wrap around mode. Very useful when making seamless textures! Another thing is that its file format contains a merged image, which can easily be extracted (.kra and .ora files are zip archives). I've used that to add native .kra/.ora support to a game texture conversion tool. .psd support took a lot more work in comparison.
Either way, once you get the tooling up and running, the next big step is actually designing a game and creating all the content. :)