How to set wallpaper on N7?

[Zell]

I think I've figured out where downloaded images from the web are stored on the N7.   It looks like the "Gallery" file is where they end up, which is fine.  Now I need to save that image from the web -- which in this case is wallpaper made specifically for the N7 -- and save it as wallpaper on my N7.  However when I save it as wallpaper on the N7, the end result I get is an image that is not the same size and picture quality as the image from the web.  What I do get is a portion of the image that is now blown up in size and distorted for some reason.  The device allows me to modify or crop the parameters of the image to fit the screen but only to a very small extent so that, as I say, I'm getting only a small section of the image showing as wallpaper.  How do I set an image off the web as a wallpaper to perfectly fit the N7's screen?