# Up to OS X 10.8 (Mountain Lion)
osascript -e 'tell application "Finder" to set desktop picture to POSIX file "/path/to/picture.jpg"'

# Since OS X 10.9 (Mavericks)
sqlite3 ~/Library/Application\ Support/Dock/desktoppicture.db "update data set value = '/path/to/picture.jpg'" && killall Dock