function setPixel(x, y, pix)
  if x >= 0 and y >= 0 and x < w and y < h then
    pixels[y*w+x+1] = pix
  end
end

i, j, x1, y1, x2, y2 = string.find(result, "(%d+),%s*(%d+),%s*(%d+),%s*(%d+)")

-- copy image

w, h = img.width, img.height
pixels = {}
for y = 0, h-1 do
  for x = 0, w-1 do
    pixels[y*w+x+1] = img.getInt(x, y)
  end
end

-- paint rectangle

x1, y1 = x1-1, y1-1 -- paint AROUND the found image
x2, y2 = tonumber(x2), tonumber(y2)

for y = y1, y2 do
  setPixel(x1, y, 0xFF0000)
  setPixel(x2, y, 0xFF0000)
end

for x = x1, x2 do
  setPixel(x, y1, 0xFF0000)
  setPixel(x, y2, 0xFF0000)
end

More flexible than #313