As a programmer I'm going to write custom code, but can I use art I've purchased? And if that's against the rules, how about art generators like these:

https://kenney.itch.io/avatar-mixer

https://kenney.itch.io/creature-mixer

https://0x72.itch.io/pixeldudesmaker

https://sanderfrenken.github.io/Universal-LPC-Spritesheet-Character-Generator/

https://mochakingup.itch.io/cc2d

Yes, if you bought them and have all the rights to use them, then you are free to do so, same for generators!