Several people here have strongly emphasised the need for Alt tags on images. Can someone give me some insight into Alt tag text for the benefit of the blind? I know how to do them, but what should they say and it is sometimes OK to put in an empty Alt tag text.

It's obvious when the graphic is a picture of somebody or something, but what about bits of navigation furniture like arrows (which the blind won't use), logos, decorative bits and pieces - should they all be fully described, just so that voice output will tell the user they can effectively be ignored - but then if you're blind surely you can ignore all of them anyway...

I'm not challenging the idea, just want guidance on what's useful to put in the tag, because it obviously is, for some reason.

Anahata