One improvement I'd love to have here would be treating same-color background as transparent. That is centering in this situation:
Resulted in this:
Regardless of that I am getting a lot of mileage out of this :)
Let's try again! The scenario is where I have some image on a background that's not transparent pixels but the same color, like here:
It would be great if in case where all the edge pixels are the same colors then said color is treated as if it was the transparent background and centered the question mark in the middle of this 16x16 tile.
(It finally uploaded! All I had to do was use the upload image dialog rather than paste the image from clipboard)