the problem you encounter comes from the keyer. you create a new alpha but only in the size of the frame. fusion works with an infinite canvas, which isnt transparent through the key. you should use a "set canvas color" node after the keyer and bring all sliders (RGBA) to 0 (which should be the default setting) now you also create a transparent canavas around the image size and if you scale or transform the image the black border should be gone