Alt attribute for unimportant images

Are there any guidelines about what text string (if any) that should be 
specified in the alt attribute of images that only have a visual design 
purpose? So that text/ audio user-agents can ignore them?
If not wouldn't that be a nice way to make the web easier for user-agents that 
don't use images? (Something like alt="nil" or "ignore")

 - Alf Inge Tunheim