Just to set the record straight, by no means do I think that I have correct anatomical porportions. Some of my stuff I will look back and say, "ARgh, the nose is too short, or those front legs are just a tad too long."
I DO use a pencil and put it up against the photos (on my laptop screen) and find how one part relates to another , a ratio of sizes, and then go to my drawing and see if the same body parts have the same ratio.
My husband points out to me that there is software to draw outlines form photos and I could just "copy" exactly the image and then paint inside the copied outline. But, somehow, then I feel like it's cheating. Hm m m . ..
That's kind of what makes painting impressive; that it looks real but we know it was created not cloned.