These sprites were released under the Creative Commons CC0 license, which means anyone is allowed to redistribute them as they see fit. Nastanliev modified them and even credits the original artist, which isn't even required under CC0.
Hold shift to run while standing on the cube and then jump, that should give you enough speed to make it! The cube jumps are a little unstable though - we'll put out a fix for that soon! We also wanted to add in-game instructions to help out with tough maneuvers like this but we ran out of time 馃槄
