I would like to share photos that are good examples of what I am looking for to be able to create a realistic representation of your dog. I will include photos that will not work as well for your comparison.
This first photo of a dog is a great example of what I need to create a excellent portrait. The eyes, nose and the direction of the hair are all clearly seen. The head is at an angle that shows the line of the jaw. He is sitting up and I can draw down to the neck and shoulders. The picture is close enough for details, but not so close that it distorts the muzzle. 
Here is another example of two dogs that are too far away to see the detail and also the because they are laying down it is not a flattering pose to try and fade the drawing to the neck and shoulder.
A very common problem that I frequently see is that the flash from the camera is reflected in the eyes of the dog. A good way to avoid this is too take the photo outside on an overcast day. 
Expression is also very important. Besides the fact that this is blurry and hard to see detail the expression seems intense or like the dog is worried about something.
Bright sun can also cause a problem because the dogs either squint or there eyes appear to be black holes. Dogs eyes are beautiful and expressive so I want to be able to really see the color and the expression.
When you are taking your shot you may need to bend down to get almost face level. In this example the downward angle is too severe and I would not have anywhere to go to fade into the neck and shoulders. This shot is all head.
One final image that shows a great example of what I am looking for in a photo. This dog is sitting up so I can draw down to the neck, the distance is good to see the detail with the direction of the hair. I can also the eye color, shape and expression. So, I hope this helps. Thank goodness for digital camera! 