One of the biggest challenges I face when doing a dog's portrait is getting a good photo.  I rely on the person commissioning me to provide photos.  Once in a while I get a wonderful shot with enough resolution to see detail and it's relatively effortless. A lot of the time I'm working with 3 or 4 or more photos combing aspects from each trying to get a beautiful painting that looks like your pet.  Some hints: Don't use your flash if possible.  If you can photograph your pet outside that's great!  If you're looking for more of a "headshot" sometimes it's effective to have them sit and shoot down towards them while they look up at you.  That way the perspective gives their head more prominence than the body but still includes their body.  Exterior light is usually more realistic --- interior lights can skew the animal's coloring.  That's ok if I have other photos to work from that show their true coloring.  

Sometimes those "pensive" poses are the best.  Some of my favorite paintings have come from the dog asleep on the couch.  It's best to use a higher resolution on your camera so if I need to blow up your photo I can get detail.  Sometimes it's really helpful to have a friend or spouse entertain them with toys or treats while you take photos.  My shepherd, Ruby, hated the camera.  She would run and hide when I pulled it out - don't think she liked the sounds it made.  Anyway I always needed help to get her photos.  Happy Shooting!