Is this your personal Chromebook or a school/work owned machine? If it is, your network administrator could have disabled changing the desktop background, and there's no way around that.
You should be able to click the Ease of Access button (assuming you are using Windows), and pull up an on-screen keyboard so you can click on the last letter.
http://windows.microsoft.com/en-us/windows/type-without-keyboard#type-without-keyboard=windows-7
On the wall. Here's a few tutorials on how to do it.
http://www.diynetwork.com/how-to/how-to-apply-wallpaper/index.html
http://www.thisoldhouse.com/toh/how-to/intro/0,,1123142,00.html
https://www.youtube.com/watch?v=MENz-3-ydDM