I was preparing to vectorise some images from a website, and to try to save extra steps I decided not to save the images and drag'n'drop (or Open, or Import) them into Inkscape, but to see if I could not just copy the images to the clipboard (in Google Chrome there is a right-click option, 'Copy Image') and paste into Inkscape.
It worked, but the transparency in the PNGs turned to black; apparently not just a display issue, because all the raster extensions (that I understood how to use) acted as if it was a regular opaque black, and Extract/Export produced opaque black pixels also.
This would not be a problem since I was just using the image for reference, except that several of the images were originally black where not transparent, so I ended up with a solid black rectangle. I tried downloading and importing the images as I would normally do anyway, and this time the transparency was treated properly.
Therefore this is not a problem for me, it just seemed like something I ought to understand the reason behind. A quick search shows that people have had related issues in other contexts, due to the fact that transparent pixels in PNG are black by default or something (so the colour can be 0,0,0,0?).
Anyway, I thought someone here might be interested to hear about it, or knows more.
-Arlo James Barnes