A while ago, I right-clicked a JPG and selected "Set as Wallpaper". I have since lost the original JPG. Where does Cinnamon store these custom wallpapers? I do not see it in /usr/share/backgrounds/linuxmint-qiana. Here is my distro info:
Did you have to enter your password to set the image as a wallpaper? No? Then it can't be anywhere else but in your own home folder--remember that you only have permission to write files in your own home folder, unless you run a program as root (which would ask for your password, for you to authorize that). So, it's probably in Pictures? At least that is where Gnome Shell puts them.
I looked there, but it seems empty. Yet an old background that I do not want to appear, and which I tried to remove from the list of backgrounds by the appropriate GUI tool, has appeared at least twice as the screensaver and desktop background after the screensaver lock is engaged.. This is very awkward, as it contains a picture of an ex, looking rather attractive in a jealousy-inspiring way. Any idea why this might be occurring? I repeat, my /home/<username>/.cinnamon/backgrounds is empty.
My default wallpapers seem to be stored in / > usr > share > backgrounds
I run KDE but might be worth a check.
I believe custom wallpapers you set up from a photo yourself are typically just a shortcut to that photo location, so they might be in pictures, or wherever the picture was when you set it as desktop background.
You didn't say which edition of Mint Cinnamon you are using? The location I posted above is correct for Mint 17, however, Mint 17.1 now lets you add directories rather than single files and so, as cllvt stated, in 17.1 user wallpaper folders are stored as a link in the location I mentioned above.
As you have already checked and found nothing I'm somewhat at a loss to know where this image is coming from unless you manually added it to /usr/share/backgrounds or /usr/share/wallpapers.. I'll admit that my lack of ideas may be due to inexperience though and that I'm not sure of the difference between /usr/share/backgrounds and /usr/share/wallpapers.
One though I have had is that if you know the file name of the unwanted image or can get the file name by finding the image in the backgrounds GUI, it may be worth using the Nemo search function to locate all incidences of that file on your system.. just in case it exists in a location you are unaware of, then either deleting or renaming all instances to, hopefully, prevent 'backgrounds' from finding it.
Sorry this is not a more precise answer, it's not something I've ever experienced personally though.