Try my tip =D
I liked the flash because it made me laugh :D
However, when Mario falls, I have a better way of making the flash look better.
Instead of making the background change size (like you did making it wider and skinnier), try moving the background up and down only little bit. (Move it up +3 on one frame and -3 on another frame). That way, it will look more like a real "thump" when Mario hits the floor instead of making the mountain look like it's sneezing.