Lay your three dimensional letter face down and trace it onto a piece of black card stock using a white pencil. Set the letter aside and cut around the tracing using a craft knife on a self-healing cutting mat.
Cover a solid work surface with newspaper. Flip the card stock letter face-up and coat it with spray adhesive. Arrange black-and-white family pictures in a collage that completely covers the surface. Spray the back of each individual picture as you apply it to the card stock. Overlap and layer the edges of pictures to obtain the best coverage. Set aside and allow to dry.
Place your three dimensional letter in the middle of the newspaper atop a small piece of scrap wood and apply two to three coats of gloss white paint to the front and sides. Allow letter to dry completely after each coat.
Flip the card stock letter face down and apply spray adhesive to the back. Also spray the front of your three dimensional letter with adhesive. Press the letter firmly onto the card stock and weight it with a heavy book for at least an hour while the spray adhesive dries.
Set the letter face-down on the self-healing mat and carefully trim around it with the craft knife, cutting off any excess photograph edges.
Paint the front of the letter with several coats of a clear sealing medium that's safe for photographs. Wood glue, such as the kind used for decoupage, turns from white to clear as it dries. Allow plenty of time between coats for the sealing medium to dry.