Pixel art written the way it looks: pixelsFromRows takes rows of characters, one a pixel, and a palette saying what colour each one is, with '.' for nothing. A grid reads in a diff and is edited by hand, where working out the corners of a fillRect for every patch of colour is easy to get wrong.
The frog's frames are sheetFromRows, by name, laid side by side for createSpriteAtlas; hero.frames.stepA is a frame's number for its animation. The hearts and the ground's tile are one picture each, and the tile is stamped along the ground with blitPixels.