Once you have several good photographs, choose which ones you feel best represent your dog's personality; which pose you would be most happy with in a painted portrait and categorize them in order of preference. This is important, as you know your dog best. If this decision is left to me, then I might pose your dog or its expressions in a manner you might find foreign or less pleasing.
If you are forwarding prints, I prefer to work from 4" x 6" color photographs. I have good success with this size image so long as your dog's head is closely centered in the photograph. Also, the image should be a fairly good close-up to show maximum features of its coat, markings & texture. Larger prints can offer some advantage, but the added expense is not necessary. Smaller prints I am afraid necessitate the use of guesswork to fill-in details.
PORTRAIT QUALITY IS DIRECTLY DEPENDENT ON THE QUALITY OF YOUR PHOTOGRAPHS.