link to us: Is there a recommendation to provide a graphic for external linking? if so where?

Is there a recommendation to provide a graphic for external linking? if 
so where?
public sites do seem to  recently:

http://mars.telascience.org/maestroButton2.png
http://mars.telascience.org/linkToUs

The inclusion of text within this graphic may not be approved by all ~:"

Does WAI publish advice that describes an ideal linking graphic?
This one has a clear outline which is helpful.

WAI does seem very slow to take up and publish ideas around the use of 
graphics on the web.

thanks

Jonathan Chetwynd
http://www.peepo.co.uk
"It's easy to use"