Okay, so you aren't going to spend months training Spot to open the refrigerator door just so that you can take his picture, but the principle is the same. Hold a treat just out of the camera's view. Assuming Spot is adequately trained not to snap it out of your grasp, you'll get a picture of him at the height of attention. Yes, you should give him the treat once you've got the shot. But no matter whether eating is actually part of the shot or not, if you're asking particular behavior of the animal, reward it for cooperation.
Animals deserve as much respect as human subjects.
Do not offer then withhold treats. At best you'll irritate your subject, never establishing a rapport with it, at worst you'll damage its training, and infuriate its owner. If you're a pet owner, you're sure to want candid shots of your pet at play. Try to restrict your pet's movements to within a workable area so that you don't have to chase him all around the yard. Position yourself to eliminate these sorts of intrusions.