Cut the MDF to 42.5" wide, the proper width for a twin bed, and to your desired height using the circular saw. A comfortable height for the headboard should be at least half the child's height.
Sand the MDF to remove any harsh edges.
Prime your MDF and allow the primer to dry completely.
Paint your headboard the color of your choice and allow it to dry.
Mix equal amounts of glue and water together and apply the mixture to the backs of the photos of superheroes to apply them to the headboard in the arrangement of your choice. Gently "paint" over the photos with the mixture once they are on the headboard to ensure they are applied securely. Allow the glue/water mixture and photos to dry.
Spray the entire headboard with two coats of a clear gloss aerosol spray paint to finish.