Go to ASSETS folder and replace elements:
To take care about game structure, you should take into account that:
SKY.PNG is an 800 x 600 picture used as a background
START.PNG is the collective element, with a size of 32×32, you can use even 50×50 or 60×60 pixel elements. Use transparencies (not blank) as a background
PLATFORM.PNG is a 400×32 size element
BOMB.PNG is a 14×14 size element, you can use 32×32, 50×50…
DUDE.PNG is the principal character. In order to go well on changing elements, you should take into account that each character position is 32×48 pixels. And on file, there are 9 positions, 4 to move to left, one to stand by, and 4 to move to right.