You shouldn't need to have an account to view the public photos on Flickr. I was able to see the photos linked to on the NAOC photos thread without being logged in. I didn't get any kind of request to make an account. Are you sure you can't just cancel the request to join and continue to the pictures?
Also, if you ever do need to join a site to use it, you can always create a dump email account (for instance, a
gmail.com account) that you won't ever use except for activating the account. Then you don't have to worry about getting unwanted advertisements or spam at your primary account, or any linking of your online accounts.