Transparant PNG Not working

I made some sprites with paint.net, which are supposed to have a transparant background, but when i import them into unity, they all suddenly get black backgrounds. I have no idea why this happens. also, im sure they are PNG files.

Sometimes it helps in unity to go to Texture or Advanced, and tell it Greyscale from Alpha, which you can uncheck, but it will unlock Transparency From Alpha, which if you switch back to sprite, seems to keep transparency.