Most encaustic is done on a hard board. When dried (hardened) encaustics are somewhat "brittle" and due to physical movement might crack on a simple paper or canvas or matboard backing.
I'm experimenting with masonite, using either a gesso layer or a matboard glued ontop to give it some "absorption". Encasutic doesn't adhere well to acrylic gesso (what modern painters use), you have to use oldfashioned rabbit skin gesso. Nasty smelly slow awful stuff. I hope that matboard continued to be the right answer!!
So, yes, I could glue the photo down to the prepared board first. However, the fun thing about collage is that not everything is at the same height. Gluing the photo to the board forces the photo to always be the lowest level. What I really "want" is a VC paper that will accept (absorb?) wax, so that I can place the image anywhere and anytime into the collage
Hopefully everyone isn't singing that old Rolling Stones song.... "You don't always get what you want.... You don't always get what you want....." Sorry, that is all I can remember of the song (being more of a jazz and classical fan myself)