If you are taking new photos these are a few things to consider:

Lighting	+ Outdoor lighting is best but if you need to take the photo indoors, be sure to have enough light. + Take the photo in the shade if you are outdoors to guarantee true colors. + DO NOT use a flash. .you'll be sure to get red-eye and I won't know  your dogs eye color.
Your Position	+ Try to take the shot at your dog's level - eye level would be best. + Make sure he is calm and comfortable while taking his picture. + You may need someone to hold him but that will not be a problem for me as long as his face isn't covered.
Expression and Pose	+ A smile would be great unless you have a very serious dog. Try to capture his personality so that I can duplicate the feel of your pet in the painting.