I can suggest you one work around that may work for you.
In PSE you can create a Blank File that can have a white or Transparent background as you want to.
As per you posted you have the image size beforehand.
So I will suggest you to create a blank file with transparent background of the same size as of your image size and then place your image on new created transparent file.
To create a new blank file :- steps
1. Go to File -> New -> Blank File or you can hit Command + N (for MAC) / ctrl + N for windows.
you will have below window ...
After creating a new blank file now you can place your png image.
Hope it would work fine for you..