Render it (Cmd(Mac)/Ctrl(Win)-r), then File>Save last render. If you want an image larger than the viewport, go to Render>Render settings and enter a new size, then pick new window as the destination (or pick File as the destination, and click the ... button to give it a name and format without having to use the File menu). While you're in the Render settings dialogue, try dragging the slider at the top all the way to the right for a better quality (but slower) software render, or try some of the other positions for various versions of the hardware render.