I don't know, aren't the shoulders in a very uncomfortable position holding the bar? Also is it safe for the back?

Way to go guys, very impressive!

We tried holding the bar in our teeth but that didn't work out so well...

Alex, I assure you there is nothing "comfortable" about squating heavy weight. All four athletes are exceeding their comfort levels and challenging themselves in this photo.

The shoulders in that position create a secure shelf for the bar and place it squarely over the feet, where physics dictates that it must be for an efficient lift. Done correcly, yes, it is safe for the back; in fact it's very good for the back. For an excellent discussion of the hows and whys of the efficacy and safety of these lifts, you really should check out the book Starting Strength, by Mark Rippetoe and Lon Kilgore. It'll answer all your questions, and it's a heck of a fun read to boot.