I just checked out my old Photobucket account. They are more intuitive... even labeling what some of the "links" are for. That's helpful for sure.
On flickr when you're looking at a pic you should see the following menu to the right
View attachment 79249
When you click on the box symbol with the arrow pointing upward/right it will give you a new menu that you can select for FB, Tumblr, Twitter, Pinterest, etc. I think that's where you'll find the Thumbtack to get the URL to insert. That's the way I've been doing it. There could be easier ways.