Is it a historical photograph or a photograph published in a book that someone scanned and posted on line, is it a photograph of something like a sculpture? Is your paper focused on the work of the photographer, the makeup artist who prepared the model, the digital image enhancer who altered the image, the model? There is no single correct way to cite a photograph, because there are many different reasons to cite a photograph. Your instructor would be able to give you more specific advice. In general, though, the 8th edition of the MLA guide would say something like this:
Check to see if the the PDF is written by an individual, set of authors, or an organization or company. If it is not written by an individual or a set of authors, use the name of the organization or company responsible for creating the PDF in place of the author’s name. Continue with the title of the PDF, version (if there are different versions available), the publisher (only include if the name of the publisher is different than the name of the author or the title), the publication date, and the location (usually a URL if found online).