Error 10327: Can't determine definition of operator ""+""

Almost definately not.

How do you expect a 50x50 image to arrive at the same time? why did you make some massive array of unsigned? why not just a 2d array of unsigned:

type image_array_t is array(0 to 49, 0 to 49) of unsigned(7 downto 0);
signal my_image : image_array_t;
for x in my_image'range(1) loop
  for y in my_image'range(2) loop
    my_image(x,y) <= something;
  end loop
end loop;

But if you expect a 50x50 image to just magically appear, I suspect there is something more fundamentally wrong with your design than a '-' function not working. The error you had will be because you sliced something wrong somewhere (and because it is rather hideous to look at I cant be bothered to find it right now)