This example has only three steps to make door way effect. it is very interesting, follow now.
Take a picture: Capture any inside door picture of the house.
Selection: Make selection by the Polygonal lasso tool (L key) as I have done here.
Take a field picture to paste in the selection, make copy (Ctrl + A) and (Ctrl + C).
Paste: Go to Edit menu > Paste into (Shift + Ctrl + V) as I have done here.
This is your done.
Recommend the tutorial