I was able to successfully do this. I simply opened the original image, altered it, "saved as" as a .png. Then I renamed it "left.png" or "right.png" (saving the originals eslewhere for safety's sake

). It worked just fine. Just make sure that the image you're working with has a transparent background.
Also, if you're not already doing so, I would begin with the original image and go from there.