Q: Dock won't launch, desktop image missing after I installed OS X update 10.10.5.

Try doing a Dock restart using the following Terminal command:

killall Dock

Log out/in and test. If that doesn’t work, you need to look in your user Library/Applications Support/Dock for the .db. Use the Finder “Go To Folder” command. Enter ~/Library/Applications Support/Dock.  Move the .db to your desktop.

Then try a dock reset.

Applications/Utilities/Terminal enter the command

killall Dock

Log out/in test. If it works okay, delete the .db from the desktop.

If the Dock is the same, return the .db to where you got them from, overwriting the newer ones.

Do a backup, preferably 2 backups on 2 separate drives.

Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J.  When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder.  Select Library. Then go to Preferences/com.apple.finder.plist and com.apple.desktop.plist.  Move the .plists to your desktop.

Re-launch Finder by restarting the computer and test. If it works okay, delete the plists from the desktop.

If the same, return the .plists to where you got it  from, overwriting the newer one.

Thanks to leonie for some information contained in this.