Once you have your key terms, you can easily navigate search engines, databases, libraries, archives, and other resources for related sources to contextualize and provide more information about the photograph(s) you have viewed. These sources should be credible, such as websites that end in .org, .edu, or .gov, just to name a few.
Once you find a credible source, read through it thoroughly and take notes, just as you would when reading a chapter of a textbook, an article, or section of a book for a class. This way you can easily refer back to your notes when it's time to synthesize your research and make conclusions about the photograph(s).
While reading these resources, make sure to consider who wrote them: was it the same person who took the photograph? Was it someone who witnessed the event or knew the person they are writing about? Consider the purpose of the source, just as you've considered the purpose of the photograph. Depending on their purpose and perspective, the person(s) who created the source you are analyzing may be contributing to the same narrative as the photograph or offering a different point of view. Conducting research also may allow you to see a photo in a different light and challenge your initial assumptions about the photograph's subject and purpose. Ultimately, sources may contradict one another and, in those cases you will need to look at other sources to weed out the similarities and differences and the facts from the bias.
The more sources you examine, the better your understanding of the photograph and the event or person to which it relate. Always try to find sources from multiple perspectives. If you're examining a photograph of the Battle of Gettysburg, for instance, that was taken by a Union photographer, you may want to look at his personal account of the events, if one exists, as well as the accounts of soldiers from both sides and anyone else who might have been present when the photographer took the picture.