I chose to modify one of your headboards by only doing step 1-4.
We made a queen headboard and I wanted it slightly taller than the plans suggested so I added 12". I used 3/4" MDF as the back cut 60" w x 48" H, 1x4s for the sides and top and a 1x3 for the bottom. I glued and nail gunned each piece to the edge letting the sides extend down for the legs of the headboard.
I filled the nail gun holes with wood filler. The piece was primed, then painted with 2 coats of color, and then the photos were Modge Podged onto the interior of the headboard touching and overlapping. As my daughter gets older we can add more photos or I can make a fabric insert to go over the photos. She loves it and I'm happy we could do something different.