width = 160;
height = 120;
mask = zeros(width, height);

center_x = width / 2;
center_y = height / 2;
radius = min(width, height) / 2;
radius2 = radius ^ 2;
for i = 1 : width
  for j = 1 : height
    dx = i - center_x;
    dy = j - center_y;
    dx2 = dx ^ 2;
    dy2 = dy ^ 2;
    mask(i, j) = dx2 + dy2 <= radius2;
  end;
end;

picture = randn(width, height); % test image :)
masked_image = picture .* mask;
imagesc(masked_image);

print out.png