All figures must have a basic caption consisting of a figure number, a short statement of what the figure represents, and an attribution (if the figure is from a source other than the author).
Put long discussions of a figure within the main text. Generally, captions should be complete sentences; aim for consistency within a publication.
In manuscripts (the undesigned text document), each caption should be placed in its own paragraph, after the paragraph that contains the in-text reference to the figure. The designer will style captions during the design phase.
In a designed publication, captions will be placed below or to the side of the figure, depending on space limitations and size of the image.