Вот мой неуклюжий обходной путь:
let copy_image image =
let w, h = Image.width image, Image.height image in
let copy = Image.create ~kind: `FASTEST ~visual: (Image.get_visual image)
~width: w
~height: h in
for x = 0 to w-1 do
for y = 0 to h-1 do
Image.put_pixel copy ~x:x ~y:y (Image.get_pixel image ~x:x ~y:y)
done
done;
copy