If you add an ID to a figure, you must have a caption. The caption is needed in case a cross reference is made to the figure's ID. In that case, the caption becomes a hyperlink to the figure.
Here's the markup for a figure with the ID my-big-picture.
<figure id=my-big-picture entity=big-picture-TIFF> Here's My Figure <\figure>
Here's a paragraph where the phrase "easier than ever" has been assigned the ID easy-spot:
Getting help is <location id=easy-spot> easier than ever<\location>.