Is there any advantage from a Google seo point of view if an external website use my images in its pages in this way: <img src="http://www.example.com/images/pic1.jpg"> where example.com is my website. Or is it only using my bandwidth? :)
To be blunt, I think it's mostly just using your bandwidth. I'd like to say that it gives you greater Google image exposure, but from what I've seen, it's just as likely to give the hotlinker more exposure.
On the other hand, it's probably a signal of some kind, and in the days of pinterest perhaps there's something to that. But I wouldn't hold your breath at this stage. You'll likely find that most people here who know how are blocking hotlinkers ;)
I think everyone starts out doing this bit automatically:
(minus, ahem, the double escapes) and then after a few years you whap yourself upside the head realizing that it isn't (www.)?yoursite.com. It's either www.yoursite.com or yoursite.com, depending on your personal preference. So if an image names the other form, it's just another fake referer.
Watch your logs closely and you'll find google colluding in hotlinking. No, not in Image Search: in Preview. If someone previews a page that contains hotlinks from your site, it will show up with Preview as UA and the offending site as referer. If you try it out you may even get to see your own "No Hotlinks" or "Stop Thief!" graphic in microcosm :)