It appears that you have a Photoshop Elements question, but have posted to the Premiere Elements Forum (video editing).
I will Move your post to the PsElements Forum.
From a Photoshop perspective (I do not have PsElements, but only its "big brother"), PNG is a very common format, and Ps CS 6 (and all the way back to about version 5, IIRC), Opens them just fine.
Can you Open those PNG files in say a viewer? I worry that they are corrupt.
Still, in the PsE Forum, you will get great assistance.
Are you trying to import the files into the pse 11 organizer or open the the files using File>Open in the pse 11 editor?
What happens when you try?
What operating system are you using?