Your issue with the PNG file is not uncommon if you do a Google search. Just about every program - from video editing to Adobe Suite - has experienced this. I think it has to do with 8-bit PNG files, which can only have a single transparent colour. Some format differences cause other programs to not recognize the transparent bit. Ideally, it is just a matter of re-establishing "black" as the transparent colour. Hopefully there is not the exact same colour within the graphic.
24-bit PNG files support alpha channel, which allows gradient and partial transparency. These tend to be better recognized across different platforms, but produce larger file sizes.