Writing a caption is simple. All you need are the six Ws who, what, where, when, why and how (OK so the w in how is at the end).
The caption under the photograph of the Qosarif mosque in Kazan uses most of these but if you click on the picture you will see a different version.
Why Different Versions?
If you are producing a general caption for a picture, e.g. an image that is being sold by a stock agency then include as much information as possible. An editor can always remove surplus information but it is difficult to add.
If you have a specific market or publication in mind then tailor your caption to that audience.