For example,
here is a page with many photos of this host

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 :

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.

LOL, nothing happened, sorry :S haha.

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