Comment 51 for bug 1066113

@Mateo:
If you want to avoid that behavior, all you have to do is not use LightDM Variety feature, and, instead, set the right permissions to all the pictures you want Variety set in LightDM login screen, something like as follow:
# sudo chmod 777 /[your_pictures_path]/*.*

Note: And everytime you copy a new picture/wallpaper inside that path you will need to give that picture the right permissions/maybe full permissions like chmod 777.
chmod 777 gives full permissions to a file, a folder or a list of files. I think you should learn about giving file/folder read/write/execute permissions.

Bye.