First, lure it to you by using a Pokedoll/plush, so it stays close to you and not roam around. Leave the doll on the ground and don't move for 2-3 minutes. You'll eventually fall asleep, Munna following suit, and that's your chance to take a picture.
*Edit* This was also already answered in the "Answers" section.
"Hello. My Name is Inigo Montoya. You killed my father. Prepare to die." - Mandy Patinkin, [The Princess Bride]