A similar question was recently asked and there were many informative answers given by knowledgeable attorneys in this area. I've included a link to the question and answers below. That will provide you with some guidance.
Bottom line - Your work must be "transformational" and not merely a "derivative work" in order to avoid a copyright issue. Those are terms of art in copyright law and you will likely need to see an experienced copyright lawyer familiar with the case law. He or she can advise you as to whether what you propose would likely be considered transformational or a derivative work.
Best of luck!
This answer is for general information purposes only. This communication does not constitute legal advice, nor does it form an attorney-client relationship.
Copyright infringement. 17 USC 106 http://www.law.cornell.edu/uscode/text/17/106 That's a derivative work you would be drawing. If you make enough changes then you "transform" the work into a different work not a derivative. Derivation or transformation. Inspiration or Imitation. That's the question and the devil is in the details. You can avoid a copyright infringement issue by doing entirely creative original work. You cannot avoid copyright issues if you are an artist, as copyright subsists upon original works of authorship expressed in tangible form 17 USC 102 http://www.law.cornell.edu/uscode/text/17/102 You want a copyright issue, namely the issue of protection to you for original artwork.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
You are planning to make a "derivative work" as this term is used in copyright law. You are transforming the picture into a different medium and may have your own copyright protection in the original elements. This is a complex area of the law, but unless you are planning to make a near perfect reproduction and sell it on a grand scale, you probably have little to nothing to worry about.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
This may seem obvious but sometimes the obvious is ignored: You incur no legal liability by simply drawing the work that you describe -- liability arises only if you publicly display or distribute that work. So using another's photograph to practice drawing is not unlawful nor is keeping [or even displaying] your drawings in your own residence.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.