There are two ways to add a caption to a photo: when first uploading it and after the fact in the media library.
In most cases, you have the option to format the caption using emphasis, alignment and links. (Some websites use their own preset formatting) Once added, the caption is attached to the image so if you use the image in more than one place on the site, it will have the caption.
Editing an existing image
Go to the media library (in the top navigation) and search for the image. Click ‘edit’ and it will give you the option to add alternative text, a caption and title.
When editing a page, you can double click on an image to access the media browser options. Click the ‘edit file’ button under the image to add alternative text, a caption and title.
There are some features of the site that will require you to edit the source code. Check out the HTML page to learn about these features and instances you must edit the code.