alt and longdesc

what is the correct way

[fwordboy]

I am slightly confusing regarding alts and longdescs.
alt is a replacement for an image so how what should go in an alt.
alt="an image of a red flower"
surely this doesn't mean anything nor does
alt="a screenshot of such and such"

so is it okay to usealt="" when the image honestly adds nothing except visual stimulation. With XHTML 2 removing the img tag how will we add images purely for visual appeal (as with XHTML2 all images must have an equivalent e.g. <p src="img.jpg">blah blah blah</p>

My other quest is find out more about longdesc. If I describe the picture in a paragrpah surrounding the image is a longdesc necessary? Or should I id it and link to it (#longdesc). Should I put all my longdesc on one page all setup with id's (so they can be linked to) and then links from them back to the original page?

Any help about this would be great.

[mattur]

There's a good article by Simon Willison called Writing good ALT text [gawds.org] with links to more info. HTH.