Attr.DefaultImageAlt
TYPE: string/null
DEFAULT: null
--DESCRIPTION--
This is the content of the alt tag of an image if the user had not
previously specified an alt attribute.  This applies to all images without
a valid alt attribute, as opposed to %Attr.DefaultInvalidImageAlt, which
only applies to invalid images, and overrides in the case of an invalid image.
Default behavior with null is to use the basename of the src tag for the alt.
