"Captions under images are read on average 300% more than the body copy itself". http://blog.kissmetrics.com/shocking-truth-about-graphics/
My research tells me this finding comes from David Ogilvy, a famous advertiser. From my personal experience and from watching users through ClickTale, I would say its true and users will pay more attention to (good) images in the right context.
Does anyone know of any scientific or psychological reason that people like to read image captions and WHY they get read so much? What about them draws so much attention?