"Here are some interesting images that might make a nice desktop wallpaper for you," is how Jim Hamm begins his latest email. "I think I might go for the Andromeda image. On the other hand, hmmm?" http://osxdaily.com/2013/06/28/9-high-res-space-wallpapers/
They're gorgeous, they're free. But how do you put them on your computer?
Jim to the rescue. "Just right-click the image and 'save image as' or copy image and paste wherever you want. You save each image separately and then select the one you want for your desktop wallpaper. You also can have the wallpaper change ever so often, if you want, in System Preferences."