No problems during the install?
As for me there where about 18 of them in there when I installed.
I started of thread sharing some of my photography as wallpapers.viewtopic.php?f=20&t=117203
Feel free just left click on image then right click set as wallpaper from firefox.
look upper left corner on wallpaper dialog for fitting to your resolution as they are 1680x1050
might have to choose best fit,stretch,etc from the pull down menu.
Firefox saves in root of /home folder.
Also can browse Google images search for Mint wallpapers and click on full images and right click set as wallpaper also.