Hi,
On this matter i have a thought... I don't know if sharing a link and load the image is the best idea because you are watching at a preview and when you click on it the user leave your website by opening another tab to see the picture...
Additional to this, if that picture will be available "tomorrow",well, you don't know that... with this feature the users are becoming accustomed irresponsible on what they share and in time you are collecting URLs in the database...
I think it's a good idea to have a system similar to CS CART CMS ... when you creat a product and want to asign a picture to it you have the posibility to put a link and then the system download and then upload it to your product... applicated to ES that means: when a user put an url in the story stream, the system "Fetch new scrape information" and if this url leads to a picture then it will download and upload to default folder (timeline folder) or ask me were to upload... this system is more efective for the site dimension because, if you have seted a limit for uploading picture then you "force" the users to clean her stuff.
It's just an opinion open to discuss