I notice that every major SE site has its own logo. I don't know who has the power to set a logo, but I think it would also be a nice promotion to do a blender SE logo contest in the blender artist forum or something like that.
When the site is about ready to graduate, the SE team will ask a question here on our meta looking for ideas about how the finished site will look. This will include the logo and graphic design. The team will design us a custom branding which will include the logo.
https://english.meta.stackexchange.com/q/400/24168 is one site's design suggestions and you can see how it turned out.
As I said, this will all be decided when we get closer to graduation, but it can't hurt to think about it now.