They aren't, but some of them have the same round shape, your tricks will help me a lot. Yes, I created different backgrounds levels to verify the smilies look good on both clear as dark colors. To cut the edges I obtain satisfactory results by erasing the white pixels and then harmonizing the edges with a fast touch of the burn tool (1 pixel size, range: shadows and variable exposure), or simply with the blur tool, in this case within a selection mask, to prevent the image to enlarge. With a bit of patience, the risults can be good.
Thanks a lot for your help!