Welcome to the forum.
What are the pixel x pixel dimensions of those PNG images. File size does not really mean anything in video.
What is your Project's Frame Size in pixels, say 1920 x 1080?
Exactly how did you convert the PowerPoint "slides" to PNG?
Good luck, and please let us know a bit more.
Back in the day, to get slides to look good on a DVD, I had to make sure that the original square pixel size was 720x540 and then resize them for .9 pixel ratio to 720x480. They would looked smooshed on the computer screen but would look good on the final DVD. If I didn't go through this process the slides would appear to be low resolution.
However the newer versions of Premiere Elements seem to figure this all out without me having to jump through these hoops anymore.