Découpage is the technique of applying paper to an object or wall and then applying many coats of varnish to create a lacquer finish, or one that is smooth, transparent and highly resilient. Although the term comes from the French word meaning "to cut," découpage was developed in second century China. But it was the Europeans who elevated découpage to an art form. In Italy, apprentices applied up to 15 coats of varnish to create a lacquer finish. But for a wooden picture frame, you don't have to apply nearly as many coats -- even just half as much will result in a gleaming lacquer finish.
Wipe the frame with a damp paper towel to ensure it is free of dust. Let the frame dry thoroughly.
Cut out your photo so that it looks exactly as you wish it to appear on the wooden frame. A craft or X-acto knife will enable you to make finer, more precise cuts, such as around fingers or other small details in the photo.
Optional: Apply a light layer of spray adhesive to the back of the photo, and press the photo onto the wooden frame. Ensure that you like the positioning of the photo; if not, carefully pick it up and reposition it. The spray adhesive will keep your photo in place but will not provide a permanent seal. Once you are satisfied with the positioning, put a pencil mark on the frame to note the placement of the upper edge of the photo.
Coat the back of the photo with a thin layer of découpage medium using a disposable foam brush. Place the photo on the frame and smooth it out with your fingertips to work out any air bubbles. Press the edges of the photo with your fingernail to secure them. If glue oozes out, wipe the glue with a cotton swab.
Make sure that the brayer is clean before rolling it over the photo several times. Coat the front of the photo with a thin layer of découpage medium with a disposable foam brush to seal the photo to the wood.
Coat the photo and the entire front of the frame with acrylic varnish with a disposable foam brush. Let the frame dry. Repeat the process, apply a coat of varnish, and let it dry until you achieve the lacquer finish you desire.
Run your hand over the frame during a "dry phase" to ensure that the surface is perfectly smooth. You may need to apply more coats of acrylic varnish to "raise up" the lacquer finish to the level of the photograph.
Work in a well-ventilated room or a room with an open window.