You need to remove the background image of the desktop from the lock screen, while to the desktop, changed the Wallpaper.
On Ubuntu Gnome could disable the background on the splash screen using gnome-tweak-tool
A screenshot of the program (made from cinnamon)
but, as I have in Linux Mint is not Gnome, and Cinnamon, then this procedure will fail here (surprisingly setting the display mode, i.e."stretching" the image to the desktop)
Accordingly, see the solution in two problems:
1. If the analogue gnome-tweak-tool for Cinnamon?
2. Otherwise you can disable the synchronization pattern from the desktop and from the lock screen?