You can save the currently displayed image using the File > Save Image command. This works both in 2D and 3D modes. The PNG, JPEG, TIFF, BMP and PPM image formats are supported. You can also copy the current image to clipboard using the Edit > Copy Image command.
To quickly generate an image with the size of your screen (which is useful for creating wallpapers) you can enable the full screen mode and press Ctrl+S to save the current image.
Use the File > Generate Image command to generate an image of an arbitrary size, up to 4000 x 4000 pixels. You can also specify different advanced settings than the ones used in the main window, so you can use this function to generate high quality images and use fast settings while navigating.