Transparent parts of image show up as black Sometimes I insert an image (PNG) and the background remains transparent, as intended.

Some other times I insert an image and the transparent background becomes black. This happens seemingly at random, but probably has some explanation. 

How do I make sure the transparent background of an image remains transparent?

EXAMPLE

This PNG image (below) appears in Premiere with the transparent background in black.



I then used Paint.net to resize it (to 75%). It was again saved as PNG (below). The transparent background showed up correctly when I inserted this new, smaller image into Premiere.



It would seem that the only difference between the two PNG images is the size.

This is what these two images show up as in Premiere (with a white-ish snowflakes background).
The most likely thing would be that you are choosing different settings for the way the alpha channel is interpreted.  There are multiple options and some will not interpret it as transparent.  I believe there are 3 different choices if memory serves.  If it is actually defaulting to different settings, then I'm not sure what would result in that though.