Nice guide by Socks, though he left out the part about getting the link to the image.
This is what I do (maybe P-dog knows an easier way):
1. Go to the photo you want that is already posted to the internet (like a photo on Fb).
2. Right-click on the image and select "inspect element"
(Are you using Chrome? Should be the same across browsers, but if you want the directions to be exactly the same, then use Chrome.)
3. You've just opened a window that reveals all the code for the page! You're under the hood! Ooooo. The code for the image should be highlighted for you.
4. But too much of it is highlighted, so only copy the part that is in the "quotes", beginning with "https://"
; and ending with ".jpg"
And that's how you nab the link!