It uses a combination of things - context (surrounding text, etc.), alt tags, page titles and so on.
It doesn't appear as though there are alt tags on the images at the moment, perhaps the description would be useful here as that's what the alt="" attribute is intended for? Would help search engines a little, add compliancy and help accessibility.
I am not going to use the description text, though. I am going to use the title text.
The title text is more appropriate because a) it's required and it will always be there and b) the maximum length of a title field is more appropriate than a description which although can be trimmed, could then affect the context of the description.