Error when trying to uppload header to folder. Help please

Hi all ive just found this software and am loving it so far but am having a problem whilst trying to upload an image for my header i get the following error.

Unable to create directory /www/99k.org/w/h/i/whileenglandsleeps/htdocs/wp-content/uploads. Is its parent directory writable by the server?

I am sure this is a permissions error but wordpres has full read write priveledges. I am unsure what the problem is. Thanks for any help.