[Python] Code in Python Brython with moving an image

The pony image takes a box of the background from where it was originally placed with it whenever it uses the move_pony function. I want only the pony to move and not take any background with it. The oupt looks like this enter image description here I don't want the entire pony box to move but only the pony. How can I fix this?

set_size(500,400)

castle = Image("https://codehs.com/uploads/7870869b7f83173c85a485528fe3f558")
castle.set_size(get_width(),get_height())
castle.set_position(0,0)
add(castle)

pony = Image("https://codehs.com/uploads/531804f0be469050b7b3960871112b83")
pony.set_size(100,100)
pony.set_position(get_width()-150 ,275)
add(pony)

def move_pony(event):
remove(pony)
if event.key == "ArrowRight":
pony.move(5, 0)
if event.key == "ArrowLeft":
pony.move(-5, 0)
add(pony)

add_key_down_handler(move_pony)`

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