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.

caBIG logo

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.