Answer questions for your audience. Think about what the reader will want to know while looking at the photo. Depending on the photo, the reader may be looking for a thorough explanation, or simply the names of the people pictured. For example, a historical picture may require the date and location of the photo, while a photo that a couple took for a wedding engagement would not.
Make sure the viewers are satisfied with the caption. Look at the photo, place yourself in the readers position, and ask yourself the questions "Who is that?" "Where are they?" and "What are they doing?" If answering the question makes sense, answer it. Answer the questions who, what, when, where and why.
Write relevant descriptions but don't state the obvious. If you're writing a caption for a photo of a girl riding a bike down the road, you would not need to say "The girl is riding her bike down the road."Instead, identify the person riding the bike and if relevant, give her destination, age and other appropriate descriptions. Always use present tense in captions.
Use quotations by taking direct statements from the people pictured to give your caption an emotional appeal. Quotes should directly relate to the action being portrayed in the photo.
Use dates, which are helpful in captions in the event that a photo is being seen for long periods of time. For example, a photo taken in the summer, but viewed in the fall may have the viewers wondering when the photo was taken.