To use a long description for images you have to add a column to the image library you want to use. Read the http://bks.1eeurope.ch/en-US/examples/Documents/Erkl%C3%A4rungen_zur_Accessibility-Checkliste.pdf document to gain information about usage of the longdesc attribute on html images.

Choose Create Column under the lists settings and add a multi line text column named 'LongDesc' to your list. Make sure that the Name of the Colum is 'LongDesc' (case-sensitive) because the aspx-page indicated as a parameter for BKSImage is displaying the content of this field for each particular image.