A big part of your learning process should be to copy stuff as much as you can. See what works for you, and it's a faster way to improve your technique. Simply, as said by everyone above, make sure you don't cross the ethical line of claiming it as your own, or selling it.
Concerning your drawing above, I think that although the colors and shapes are moslty in place, the brush you are using is way too hard to blend together the smooth and subtle changes of value in skin color.
Try doing this: choose a very soft brush (with or without textured patterns, as long as it's not hard), set pen pressure to opacity (if you have a tablet), and blend every colored zone on your drawing on the next one. Keep a finger on the "alt" key to quickly switch to the color picker tool. You should be able to quickly switch to the appropriate color for a smooth blend, which should increase your skin rendering a lot.
Hope this helps, keep at it!