Getting a photo of your pet that will be a great reference for a portrait painting is surprisingly easy if you follow a few simple guidelines. What actually works best is to start with photos that don't work.
Photos that look down at your pet. While we're used to looking down at our pets, a portrait that is downward looking rarely looks good and often makes the animal look subservient and puny; not the object of getting its portrait painted!
Photos with flashed eyes. While I can work around such photos sometimes, I can't really convincingly supply visual information that's not there in the reference. We artists are only as good as our reference photos: "garbage in, garbage out".
Photos that are very dark or backlit so that the pet's face is in shadow.

Over the years I've developed a foolproof method to getting great reference photos:

1. Use a camera that allows you to shoot without a flash so that you can avoid that devil dog look!

2. Seat the dog in front of you and either get down on the floor so that the camera is at head level or seat the dog on a table or platform.

3. Have a friend stand behind you and a few steps to either the left or right of your shoulder. Give the friend a treat or a noisemaker. When the dog's attention is on the friend, its nose will be off-center (highly desirable), its far eye will still be visible, and its ears will be at attention. SHOOT THE PICTURE NOW!!!