One of the most important things for a portrait artist is a good reference photo if she is an artist who works from photos. I prefer to take these myself, but sometimes this isn't possible. This was true in my most recent portrait.
After looking through the available images, which were few thanks to Hurricane Katrina, we settled on this one as having the best facial expression and having the most visual information. I scanned it into my computer and went to work with photoshop, lightening it to try and bring up more detail. With the face, I was fairly successful. With the tree bark, not so much.
Sometimes I don't put any background in at all, but I didn't feel that was a good choice for this image since she was leaning back against the tree. my solution was to go into my back yard and photograph one of my own trees with similar bark so I could get an idea of what the tree would look like if the photo was more detailed.
I placed it roughly with Photoshop. It didn't have to be seamless since it was only for visual reference. I could do all of my other work from the first photo. Tree bark, I discovered, takes a long time to draw, but I was happy with the finished product.