I've installed Ubuntu 12.04 Beta2 today, and I stumbled upon this graphic during installation.
Does anyone know, where in system I could find it and extract it, to use it i.e. as a wallpaper?
You most likely will not find it on your system. It is included on the live cd (as part of the set of images that are shown during installation). It will be a binary (to save space) so you will need special tools to extract it but the one on the disc is probably going to be 700x470 (so the same size as the image you already found).
If you want that as a wallpaper you probaly need to make it yourself (or someone else has to make it ;) ).
I took these files
and made this image that looks CLOSE to the one in your picture, but not exactly. unfortunately, I couldn't find the exact one you have. I hope fits your needs.