For example,
here is a page with many photos of this host
http://www.flickr.com/photos/bellayt

Just click the photo you are interested in, open it in a new page.
Then on the top address bar, it's right the photo url, easily to get it :)
here is a flickr photo url eg list :
http://www.flickr.com/photos/bellayt/6688707109/in/photostream
http://www.flickr.com/photos/bellayt/6676688021/in/photostream

http://www.flickr.com/photos/bellayt/6676687823/in/photostream

Did you find something similar along the flickr photo urls? any law?
Yeah, only the number differs, 6688707109, 6676688021, 6676687823, they are just the photo IDs on flickr.
So, if you simply change the number by "subtract 1 or add 1" it will show another photo.
Ok, now, let's try it. See what's on the url.
http://www.flickr.com/photos/bellayt/6688707108/in/photostream
http://www.flickr.com/photos/bellayt/6676688022/in/photostream
http://www.flickr.com/photos/bellayt/6676687822/in/photostream

LOL, nothing happened, sorry :S haha.

Just read this like a joke, happy everyday ^_^

bella
budgetgadgets.com