>>6351847Gimp.
Scale image to 1080 (bottom value.) Copy image.
Make new, 1920x1080. Paste into.
Color picker, purple or whatever from background.
Bucket fill, both sides. Bucket fill values around image, upping threshold until it fills it all WITHOUT touching the main creature images (ctrl z, to undo, lowering threshold again of bucket fill until its not damaging main image.)