With HTML, every element has a rectangular bounding box. This is not generally a problem, except when layering more complicated images, of which some are links.
Above is an example from a site I worked on recently, sevenpsychopaths.com. The bounding box that triggers the link is highlighted in blue, and you can see that it extends from the actual imagery itself by quite a bit.
In response to this, I wrote this quick bounding box test using SVG:
To create this, I followed this tutorial to wrap a bitmap image with a path in Illustrator. I then saved that document as an SVG and dropped the SVG into the HTML document.
I don't tout this as the be all end all solution for bounding boxess in HTML, but I do think it offers an interesting solution for more dynamic interactivity!