Judging by the error code, BackImage, PlayerImage, EnemyImage, and Pop are all nonexistent classes.
If you have created those classes, then they're not linked.
Alright, i've linked them. now im left with the "Pop". The pop is a sound so i dont think im able to solve this now.. how do i insert a "pop" sound inside actionscript in the first place?
BTW, don't use capital letters in directory names, variables, functions or instance names. It's a really bad habit to get into, and it violates established AS3 naming conventions. (basically, makes your code and error reports difficult to interpret)