It seems that you want a picture within a picture. If that's the case, place the png on the overlay (higher) track, at the position that you want it to be seen.
Click the chevron to the right of View button and choose Clip and Background so you can see them both. 
Click the fx icon in the png clip. Click the large green + sign in the toolbar.
Use the position, crop. scale,, etc. effects until the png looks right. The lower track background will show through wherever the png doesn't block it.