first you should go to this site:
http://linuxmint-art.org/
then choose a wallpaper.

Next  click on download

after that you see a picture. right click on picture andclick on save image as ... and save it.

right click on desktop and click on change desktop background.

click on add and go to the place that save your wallpaper and double-click on it.

your wallpaper is add on background.

finaly click on close and enjoy.