To Display alternative text for an image use the following wiki markup. The alternative text is not rendered by the Firefox browser.
!caBIGlogoR.jpg|alt="caBIG logo"!
The result is as follows.
For MediaWiki, the syntax for alt text is:
[[Image:ImageName.png|alt=Description of image.]]
A full description of the MediaWiki syntax for images is found at http://en.wikipedia.org/wiki/Image_markup