How Do You Know if You Have the Right Imagery for Your Ads?

When working with clients, how do you know ifyou have the right imagery for them? And how important is it, like, getting custom photos from a company, versus may be using some stock photography. One of the things that is commonly important when it comes to facebook and instogram ads is the visual side of thingsd by that, i mean a picture and what's on it, and what's the size going to be.