Yes you do have to sign up, although it is free.
The following instructions will hopefully help, please feel free to post corrections if it doesn’t answer the mail.
First, go to www.flickr.com for sign-up link, it will walk you through the process to obtain a free account where you can post your picture (and other ones :*)).
2- Once you have an account, upload your photo to the account (there is an upload photos link).
3- Once your photo is posted, join the group by using the link below:

and clicking on the “Join this group” link on the right hand side of the page.
4- Return to your photo by clicking the “You” link at the top of your page.
5- Select your photo(by clicking on it).
6- To add the picture to the group- Select the icon above the picture to “send to group”. Clicking on that icon and will show you all the groups you belong to (you should see Appalachian Trail Completion Photos in the list), select the group and it will add your photo.
7-You know that the photo was added if on the right hand of the photo it says “This photo also belongs to Appalachian Trail Completion Pictures”
8- You will be able to see your photo at the at_completions link along with any other photos posted by accessing the direct link: http://www.flickr.com/groups/at_completions/
You can of course also see it by searching your Groups Link.
I know that probably sounds very cumbersome, but it takes maybe 15 minutes (depending upon the upload time). I tried to capture all the steps but if you have any trouble let me know.
I just wanted to create a place where hikers would be able to view “year books” - hopefully it will work out…we’ll see.
Thanks for your interest Bluebearee!
Courtney
Courtney