I actually asked that question to David many months ago. He told me that he bought it on Shutterstock. The one he sent me is this one (stock photo). But there is also a vector version.
So considering that it’s proprietary (not under a free/open license), I don’t think it’s the best candidate for using as the logo of the SAFE Network.
So far I’ve been using the MaidSafe triangle logo in the projects I am making since it’s pretty much the only option available, but I agree it would be good for the SAFE Network to have a different logo than MaidSafe. And we might want the logo to be something else than an impossible triangle.
I also like the “Privacy. Security. Freedom.” banner and I don’t consider it to be a logo.
As for using the safecoin logo for referring to the SAFE Network, I actually think it’s an idea worth considering. It would make sense that the logo of SAFE Network would be a “S”. But then again it would be nice if we had other options to choose from as well.