If you've determined that the image you want to use is likely protected by copyright, your next step is to identify the rights holder and ask for permission to use the image. http://copyright.princeton.edu/obtaining-permission has some suggestions on where to start.
ProQuest's Copyright Guide (for grad students submitting their dissertations) includes a sample permission letter.
Your publisher will specify the image format and resolution they require. Often it is best to pay for a publication-quality image supplied by the rights holder or taken by a professional photographer.