The important element is using it as reference and outright copying the image. The latter can be useful for studying purposes, but not when it comes to actual creative art, and can be stifling if you adhere too much to the information in the source material. Fanart is often a victim of this, especially when it's a recognizable piece of artwork that's being copied. Using it as reference, on the other hand, is taking the model (if you're lucky enough to have one) or the photo(s) to fill in the information you need to draw a believable image. Artists have been doing this to some degree for ages. Even Norman Rockwell used photographic reference for his paintings, but he wasn't slavishly tied to them. He often knew when to exaggerate features or what to eliminate. For one of many articles available on this topic, check out this post by James Gurney (and check out the rest of Gurney's blog and the related book for more information about this), and this recent NPR Slideshow about Rockwell's work.
Getting to the point, I've been a little random lately about using reference. I need to do it more. I recently did an exercise with a mentor over at CA.org about just this. I drew the below image of a grande jete from imagination and vague memory. There are issues with the rear leg, tangents of the near hand and body, line of action, etc.
I was then to follow-up with a drawing based on the original but with some reference applied. I opted to use a mix of images from books I had (surprisingly very little grande jetes), online videos, and images found online. Below are the quick sketches I did based on them.
And lastly, I used a mix of reference and the sketches to work out the final image. Made some changes to the pose, such as how the leading leg is usually away from the audience and fiddling with the arms. Much better, but couldn't quite avoid tangents (they kinda meet at shoulder-neck-shoulder). Still, the line of action is a little better, and the legs are stronger.
