I like seeing a photo on my iPhone screen when I'm receiving a call. However, I don't have photos associated with all of the people I speak to most frequently. Luckily, it's an easy thing to do!
In the Photos App, open the image you want to assign to your contact and select 'Assign to Contact.'
After selecting the contact, you'll get a chance to move an resize the photo.
That's it! Now whenever you call this contact, you'll see the picture you have assigned to the contact.