There has been an issue with wallpaper not being set after a reboot, and it keeps popping up, so here is the cure.:
This has been a problem for a while. Open filemanager to your /home directory, and .config> pcmanfm> There will be one folder, and two files.
The folder is "default", the two files are "main.peppermint.ice" ( or One) and "pcman.config".
Delete the "pcman.config" file because it is a duplicate file. The real pcman.config file is in the folder "default". Keep the one in the default folder. That will keep your wallpaper setting as you have it the next time you log out, and back in.