I've read that photographers add a link to their business web site but is it also allowed or ok to add a link to the image description that refers to the image at a stock agency?

Is flickr not happy about that since they work together with Getty?

Here it is (question number two)
http://www.flickr.com/help/faq/search/?q=link

Members are allowed to link to an external site where they show their photography portfolio and talk about their services. A single link to this site without sales verbiage under your photos is allowed. But we ask that you not link to a site with a shopping cart or that is essentially just selling prints.

Other commercial links are still not allowed on photo pages since, as noted in the Community Guidelines, Flickr is a personal photosharing service � not a site to sell or promote your products, services, or yourself, whether you�re a commercial entity, non-profit organization, or an individual.

"Don�t use Flickr to sell.
If we find you engaging in commercial activity, we will warn you or delete your account. Some examples include selling products, services, or yourself through your photostream or in a group, using your account solely as a product catalog, or linking to commercial sites in your photostream."

We don't allow commercial links on photos, sets, or other places in the photostream. We do allow you to have a commercial link in your profile � which is where people would go to find out more about you.

I knew that links from photostream are not allowed - didn't realise that links from the profile ARE - that's interesting...