It could be that the file associations have been broken by uninstalling the previous version after installing CC2018. This is an old problem where the uninstall takes all file associations down with it.. Unfortunately the CC installer does this by default, unless you go into "advanced options" and uncheck "uninstall old versions".
Try to reset the file associations, either in Bridge or Windows. If that doesn't help, uninstall Photoshop and reinstall.
For the future, if you don't want to keep old versions, uninstall them prior to installing the new one.
Thanks! It sounds like you're on the right track. I don't mind getting rid of the old versions. But it sounds like I just need to figure out which app runs the wallpaper and disassociate it from JPG. Any ideas?
The real problem, I suspect, is that jpegs don't open automatically into Photoshop, as they should. This is the file association.
Try Bridge > Preferences > File Type Associations first. Check your other file types while you're there (psd, tiff, png).
The first thing I did was set the file association. When I double-click on a single file it does open Photoshop. However, when I select multiple files and hit enter, it still changes my desktop wallpaper.