The LTSP login screen is provided by LightDM. LightDM supports themes and they can be found in the /opt/ltsp/*/usr/share/ldm/themes folder.
We recommended to chroot into the client image before changing stuff in the /opt/ltsp/ folder (for example to preserve the context of symbolic links).
Once you are in the chroot you can change the default theme changing the symlink called default:
ln -vsf /usr/share/ldm/themes/lines /usr/share/ldm/themes/default
Simply reboot your clients to apply changes. The standard LTSP configuration allows to do this sort of changes without the need or re-building the image (thanks to NFS).
* https://help.ubuntu.com/community/UbuntuLTSP/LDMThemeChange (note that Ubuntu does not uses NFS as default)