There seems to be an issue with transparency with an image. I've read the FAQ - the how to - the shading all to no avail. I fully understand the png-8, gif format and have had no problems with other software. I have a logo that is png-8 I KNOW it is transparent.. I can drag i can load and the background is black - I perform a transparent shading on the color black and then there is a white rectangle left behind. This should not be this difficult. I'm of the opinion that this is a bug.
If you are dragging a PNG image onto your work page and it shows a black background, then your PNG file format is not PNG-8; it is PNG-24+alpha channel or PNG-32. SS displays the transparent background of these two PNG formats as black (and this has been reported before). But the image will display as transparent when it is published as long as you do not do anything to alter its format. So do not use the shading tool to alter the transparency nor resize it as this will destroy the transparency layer.