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I typically just add a prefix and then whatever the image is. Pic related. Prefix will be something like minimal or mini for a minimal wallpaper. Something game related would be like game-TES.jpg, Linux walls would be linux-space, linux-gnu, etc... Other things that don't fit in a specific category I'd just give a relevant name (i.e. field-of-grass.jpg or something). I also just color-code certain things, if something is obviously color oriented. To me it's just more obvious to search what the image is about or a prefix of what it is to find it.
That's the exact reason I never leave images as their download name so I can actually find them later or so they are somewhat organized.
Also things that I know there will be a lot of or are a specific category I just make a new folder within the wallpaper folder i.e. pictures >> wallpaper >> xxx.
Some people also just organize every wallpaper category in folders but imo that's a pain in the ass.