The photo cannot be used without getting a written permission from the author as the Creative Licence under which the photo was placed on Flickr is the Attribution-NonCommercial-NoDerivs license. Using the photo to promote a paying event would be a commercial activity, which is not allowed under this license.
Content that is not subject to copyright belongs to the Public Domain. This means that anybody can use it and in any way they like. Note though that different countries have different definitions of what is subject to copyright and what is not, and the different ways in which copyright material may be used. The copyright licensing agency in your country will be able to provide more details.
The image you have found can be used in any way you like provided you give credit to the authors. This license is called Creative Commons Attribution license and it is a Free Culture License!
Find out more about Creative Commons licenses here: