i didn't! all the tiles i used came with knytt stories. but they'd be pretty easy to recreate, since they're just solid blocks of color.

just make a "tilesets" folder in your knytt story folder, copy tileset0.png from the "data/tilesets" folder, and draw whatever tiles you want. generally, if you have anything in your own knytt story folder with the same name as a file in the knytt stories data folder, the game will overrule the file that comes with knytt stories with the one you made. so, for example, to make the background in this story (which is a pixelated image of FLAMES from a google image search) i just saved it as gradients/gradient0.png and set all the room backgrounds to gradient 0.