Set up two sawhorses and lay the door across the top of them.
Use a reciprocating saw to cut out the portion of the door that you want to set pictures in. If the door has panels, cut around the panels, for example.
Turn the door over. Measure the width and height of the back of the hole. Purchase a piece of glass that is 1 inch larger than the width and height of the opening. Set the glass in the back of the hole. Purchase a piece of 1/8-inch-thick plywood that is equal to the dimensions of the back of the hole.
Place a picture on the glass so it will show through the front of the glass when the door is standing upright.
Screw picture frame turn buttons every 6 to 8 inches along the edges of the back of the hole in the door.
Place the piece of plywood over the back of the hole in the door. Position the turn buttons over the plywood to secure it in place.