I think dA must've derped somehow on the full-res download version; there's a strange jagged effect around all the lines that border a transparent area of the image, kind of similar to how something would look if it were "cut-out" of a larger image.
This idea is so chuffing brilliant I can't even begin to describe it. I felt the same way with the original Pinkie!Celestia pic you did; the premise just plain works, I could easily see this working as a real episode of the series.
Looks really good. Though with Derpy and Raindrops raising Pinkie, I can see this ending ... badly. Then again Derpy seems to be paying attention at the moment, so maybe this little shenanigan will last until Celestia shows up.