The HTML standard allows images to have a link to an additional file with a long description with the "longdesc" attribute. In addition there's a proposed new ARIA attribute "aria-describedat" that's similar.
Normally these attributes are only exposed to users who have screen readers or other assistive technology installed that knows to look for these.
This extension puts an item in the context menu so you can right-click on any element on the page and open its long description. It also gives you the option of adding a colored border to images with a long description so you know when one is available.