Neat game, although it feels a little too much like a chore. People who have worked with graphics programs may have done this sort of thing from time to time. It reminds me of dragging cropping rectangles to make the picture just the right dimensions.
To The-Lizard under me, the y axis isn't necessarily inverted. It's only inverted by the standards used by mathematicians drawing graphs. In terms of computer programs like Photoshop or Paint or Flash or whatnot, y values increase going downwards on the screen. So when it comes to pixel hunting in a program like this, you would expect 0 to be at the top rather than the bottom.