There are several ways to create a photo collection - http://www.geograph.org.uk/article/Content-on-Geograph
Marked list - see http://www.geograph.org.uk/article/The-Mark-facility
You can save this as a private list or use it to build a Themed Topic or Article.
Themed Topic - a discussion area where you can gather photos either alone (by locking the topic after each post) or collaboratively.
Article - see http://www.geograph.org.uk/article/Help_on_formatting_of_articles for help in creating articles which can also be a sole effort or opened up for collaborative editing.
Articles are indexed by google (and other search engines) and are visible to registered and non-registered users of the site.