Unless you want to be accused of plagiarism, you need to properly credit the photographer any time you use their work. Include the person's name, as well as other information, as dictated by the style guide to which you adhere. Depending the institution with whom you're working, you'll follow the image credit guidelines of either the Associated Press, Getty Images, the Chicago Manual of Style, the American Psychological Association or the Modern Language Association.
Figure Number in the Caption
Whether you're using the photo in a book, an article, in a blog or elsewhere, start by adding a caption under the photo, and then write "Figure" followed by a number. If it's the first image included, for example, it will be "Figure 1," while the second image will be "Figure 2." Then create a "Works Cited" or "References" page at the end of the document. This is where you'll cite information about each figure in consecutive order. Start each citation with "Figure 1" and then include the rest of the required information.
Chicago Manual of Style
For all styles, start with the Figure number that corresponds to the photo. Then for the Chicago Manual of Style, write the artist's name, followed by the title of the work, if you know it, in italics. Then write the medium, such as "photographic print," for example, its original measurements and the institution where the work is formally housed. Then mention where you obtained the work by writing "Source:" or "Available from:" and including the name of the person who provided it, or the URL where you got the image. Each individual item should be separated by a period.
To follow APA style, write a caption that tells the reader to whom the image is copyrighted and a statement of permission, such as "used with permission from John Smith," for example, as well as a Figure number. Then create a reference sheet that includes full information for the photo. Include the photographer's name, followed by the year the image was created. Then write the title of the work, the type of work -- such as "photograph". Then write the date you retrieved the work, writing "Retrieved" followed by the month, day and year you retrieved it. Finally, include where you got the work by writing "From" and the address of the website or source. Each individual item should be separated by a period.
If you're following MLA style, include the following information on your "Works Cited" page. Start with the artist's last name and first name, and then write the title of the artwork, and then the year the photo was taken. Then write the name of the institution or person housing the artwork. Following that, the format is slightly different for print versus online images. For print, write the title of the print source you got the photo from, followed by the author or editor's first and last names. Then write the city of publication, the publisher, and the year, followed by the page number and the medium of reproduction, such as "Print," for example.
For photos sourced online, write the publisher or sponsor of the website or database, the medium you used, such as "Web," for example. Write the date you accessed the photo and the URL. Each individual item should be separated by a period.
Even if you're following one of the above-mentioned style guides, the image creator might ask you to attribute the work in another way, such as including information about the holder of the copyright in the caption. If you're using an Associated Press image, for example, you'll be required to include the copyright notice "© [Year] The Associated Press" in all uses of the image.
Fair Use and Purchasing Rights
Crediting a photographer protects you against claims of plagiarism -- but that doesn't mean you can use just any photo. With free photos, they should already be issued under an open, creative commons or fair use license, and if not, you should get permission from the image creator to use the photo. If you don't, the photographer could sue you for copyright infringement. If you don't have all of the information, include the photographer's name and any other information you have. If you don't know who the photographer is, don't use the photo, as that person still has the right to sue you for copyright infringement.
Another way to go is to purchase the rights for the photo. Stock image companies such as Getty Images and the Associated Press sell photos with various types of licenses, including the right to use the photo once or multiple times under "rights managed" or "editorial" licenses, among others. Each company will have its own rules about how to credit the image's creator; so get the details from the company's sales representatives to avoid putting yourself at risk of copyright infringement.