UPDATE
Pinterest is changing the game with it's updated Functionality.
Pinterest made a small, but abrasive decision today. If you edit a link within your picture after Pinning it, they don't give you the traffic you may deserve - meaning taking you completely off their "Popular" page (think delicious). Basically, whatever picture we pin, or you pin, will gain no additional traffic unless the picture resides on your site.
How would you edit a picture (so you can stay away from it):
Pin the picture
Go to Edit URL, and change the URL to a completely different link, where the picture you just pinned is not on that site)
The above will not work any longer. No more taking "advertisement" pictures with a description of the "Top 10 Hotdog Festivals" and using it to link to that article. The Picture must be on the page somewhere.
How Does it effect us:
We advise you to have the picture already on your site, and even better - already pinned in Pinterest, but at least on your site. If it's not, and we have to change the link to where your URL goes, it won't have the possibility to go viral throughout Pinterest (with out the help of our network).
With that being said, it does have a chance to go viral within our network. Since we give these Pin IDs out to our network, and basically "check these out", then you still have a chance not only to spread to 1,000 repins, but even 3,000 or 5,000. Once Pinterest has a decent algorithm in place (They're looking to hire an algorithm guru), we may be setting your pictures up for a huge spike in traffic - especially if it has to do with the number of repins + likes + comments + followers; which is where they may be possibly heading.
Conclusion:
Keep your pictures on your own sites, no matter if it's an ad for an article or not.
Videos are special - use them on pinterest too. (YOUTUBE VIEWS)
For pictures that are not on your site, you have the chance to go viral within our network (let's see if that happens when you go buy 1,000 repins from a bot)
Our Network may be setting early adopters for a huge spike in Pinterest Traffic when the official algorithm launches!