This works because full is the default internal link style for Image, otherwise the link text would be empty since there is no title, and OurBigBook would raise an error.
OurBigBook can optionally deduce the title from the basename of the src argument if the titleFromSrcboolean argument is given, or if title-from-src is set as the default media provider for the media type:
Have a look at this amazing image: \x[image-tank-man-standing-in-front-of-some-tanks].
\Image[Tank_man_standing_in_front_of_some_tanks.jpg]
{titleFromSrc}