The big question here is what DM you are using.
Assuming you are using lightdm.
Go to the image that you want to use. Right click on it and choose the copy option.
Pull up, as root, the /etc/lightdm/lightdm-gtk-greeter.conf using your favorite text editor.
Go to this line;
should be about line 12.
Get rid of everything past the "=" and then paste your "image" there. This should look like this;
Something like that. There may be some extra things at the beginning of it that need removed to leave the clean path.
Save the file and that should be that.
You may have some problems with font color on a new image.
If so that can be fixed in /etc/grub.d/05_debian_theme. If this is a problem ask and I will find the correct line and entry.