General tip on the collision, you want your "hitboxes" to be smaller then your sprites, so if your sprites are 16x16 pixels, don't be afraid to have 8x8 pixel hitboxes. It will feel a lot more fair on the players. Right now I think they are the same size as the sprites.
Love the artwork, greatly done! Movement also feels proper, and the double jump animation is on point. I can see a lot of love went into making this.
Also love the fake boxart, we should have thought of that...