Once you learned is a quite easy taks! But how to figure out how to do it? That's the problem!!
If you look carefully, there is no way of telling where this option is. You might discover it by chance.
Wanna test it? Display your contacts. You'll get some icons as displayed below:
If you move your mouse on one contact's icon e pull-down menu is activated. Then you can choose the option you need.
The possibility of opening the pull-down menu appears only after you moved the mouse over the icon. This is not good because the interface is not telling you what are the operations available until you don't explicitly select them.
While Flickr is a great idea, it seems that usability of its website is rather bad. The contact icon has no affordance that tells you how to operate it.
The concept of affordance is a basic one in Interaction Design and making explicit and intuitive the way of operating the interface to user is the most fundamental design principle one has to consider in order to build friendly interfaces.
I hope Flickr design will soon realize their interface's flaw.