Inspired by the Damsels in Distress episode of Tropes vs Women in Video Games, and by the Dad who genderswapped Donkey Kong for his daughter, Kenna swapped the Link and Zelda sprites on a Zelda ROM, so that Zelda rescued Link:
Tiles are the blocks of pixels that make up each sprite. Animation happens when the tiles are swapped for other tiles. So, to create new animation, you just have to edit the appropriate tile.
Some tips! Make sure that tile goes back in the same place where you found it, or you might end up with random legs and arms sticking out of scenery. When you open up your ROM in Tile Layer Pro, you'll see a grid with all of the ROM information displayed as pixels. It shows *all* of the rom information, so you're going to see a flaming hot mess -- at first. Keep scrolling down until you find something vaguely sprite-shaped and use the "+" and "-" keys on the keyboard to align them. But beware -- the tiles will go back to their starting alignment when you scroll and not all parts of the code use the same alignment. We spent hours hunting down the princess tiles because we didn't realize that the alignment changed from section to section.
Kenna's made the patch available.