Scratch

You just have to make the costume switching and jumping two different scripts. I'll just say the jumping key is the up arrow, because I don't really know what key you want to use to jump. So you'd have the jumping scripts activate when key up arrow is pressed, and at the same time, another script activates when up arrow key is pressed as well. Then you just have to change costumes whenever you want them to, but it'll happen while you jump.

Does that help you?

When jump button pressed
goto costume [jump]
wait until touching ground
goto costume [ground]