Board Thread:Questions and Answers/@comment-30771283-20171113094245/@comment-1327106-20171201093340

You should have all of it unlocked already if you finished the epilogue. You fill it out by completing Attack Mode missions. If you're just trying to fill out the gallery in general, I recommend using this guide.

https://steamcommunity.com/sharedfiles/filedetails/?id=754286204