sproulman,
When you took the picture of the dog, was the camera the same distance away as the camera was from the moutain lion? Was it the same camera or a camera with the same optics? If the answer to either of these questions is no or you don't know then you really don't have any way to confirm a size difference.A blue gill can appear to bebigger than a horse in a picture if the blue gill is close and the horse farther away. You can create the same effect with different optics too. I really don't think you know how far the cougar was from the camera so I really don't think you have a valid size comparison with a dog. I really don't think you can determine the weight of the animal in the picture. In order to acurately measure an object in a picture you need to know2 things. 1) The distance to the object you want to measue. 2) The distance to an object ofknown size.