Don't have PP installed here.
In Videopad a transparent PNG image placed can be placed on a track that's higher than the background. Then the BG will be visible in the transparent (checkerboard) area.
That background can be any color, image or video. In Videopad it might be a white Blank Clip, as chosen from the toolbar.
Videopad can be test driven for a brief trial period, with no obligation to purchase.