Obtain the "Snapshot" Ascension skill for Prompto as soon as you can by spending some AP for it (and you can always add the additional upgrades to it if you desire). In combat, any time you get 1 TP, go ahead and spend it on his ability to get a picture. This skill levels up slowly, but any time Prompto takes photos, it slowly increases. If you get the Camera Strap, this will also help him in his shots, though you will not know of your progress of this skill (same for the others) until you go to a resting point (like a camp or inn).