It looks as though there's room for a thin 240V breaker, but it might not be permitted by the manufacturer. What model of panel is this? I can't see the amperage of the main breaker, but I'm concerned that you already have a 50A 2-pole breaker in this box. If the supply is 100A, you'll need to upgrade the supply. I'm also concerned about all of the handle ties in this box. It looks like they're all multiwire or 240V circuits, which is legal, but odd (at least in the USA). Or is this some kind of commercial installation (which is not for DIYers)?

I missed the panel model in your post, sorry. As far as I can tell, GE doesn't make anything larger than a 50A breaker in the THQP (1/2") line of breakers. In the full-sized line, they make larger ampacity breakers, but you don't currently have enough room. You could possibly replace a couple of your full-sized 240V breakers with half-sized ones to make more room, but I'm still concerned about the supply feeding this load center.

Is that a ground wire connected to an ungrounded terminal ?
http://www.flickr.com/photos/77767815@N06/6964490475/in/photostream/

Regarding the picture in the link below:

I believe, since these look like multiwire circuits, that you can't move everything up 1/2 of a space. That would put both legs of the circuit on the same phase and overload the neutral wire.

Here is a link that might be useful: breaker moving plan?