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.