There are two ways of working from photos. One way is to work directly from your own photographs. Since they are yours, you can copy them exactly if you want. However, I don't know about you, but I don't have access to most of the animals I want to draw, and I don't feel like sitting for an hour at the zoo hoping the lion will wake up and turn around (for example). In these situations, you can use others' photographs as reference.
I must say something very important here: You cannot copy others' photographs. Everything on the internet is copyright protected. Everything. Unless it specifically says otherwise. Many people think that anything you find on Google image searches are free-use images, but this is not the case. You can use those images to study proportion, flow of fur, expressions, shadows and highlights, color, or any number of things. But ultimately, the drawing must be your own composition.
When selecting reference photos, I look for a few things. I usually have an idea in mind first, then go looking for images that will help. Two or three for the general pose, a few for markings, and perhaps a few detail shots for mouths or paws. They are meant to aid you in your drawing, not to provide an interesting piece for you to copy.