Deepin uses the LightDM Display Manager. Chances are you have both LightDM and LightDM-Gtk-greeter in your start menu, try typing them there, or else type login screen.
If you get there, you can probably establish where the login wallpapers are stored, likely the same spot as your desktop wallpapers. You can then alter it, or add your own to the storage folder and switch to that.
I would advocate taking a snapshot with Timeshift before monkeying with the settings.
I likely can't help you further than that because I don't use Deepin.
Cheers
Chris Turner
wizardfromoz