$ tar xf /path/to/xdm-slackware-theme-2017.1026-source.tar.gz
$ su
$ cd xdm-slackware-theme
$ ./xdm-slackware-theme.SlackBuild
Slackware package maker, version 3.141593.

Searching for symbolic links:
usr/bin/setxdmbg        ../libexec/setxdmbg/setxdmbg.sh
etc/X11/xdm/slackware/pixmaps/default-wallpaper.pixmap  C1-Nights-Haze-Blue-2.jpg

Making symbolic link creation script:
( cd usr/bin ; rm -rf setxdmbg )
( cd usr/bin ; ln -sf ../libexec/setxdmbg/setxdmbg.sh setxdmbg )
( cd etc/X11/xdm/slackware/pixmaps ; rm -rf default-wallpaper.pixmap )
( cd etc/X11/xdm/slackware/pixmaps ; ln -sf C1-Nights-Haze-Blue-2.jpg default-wallpaper.pixmap )

Unless your existing installation script already contains the code
to create these links, you should append these lines to your existing
install script. Now's your chance. :^)

Would you like to add this stuff to the existing install script and
remove the symbolic links ([y]es, [n]o)? y


Removing symbolic links:
removed './usr/bin/setxdmbg'
removed './etc/X11/xdm/slackware/pixmaps/default-wallpaper.pixmap'

Updating your ./install/doinst.sh...

This next step is optional - you can set the directories in your package
to some sane permissions. If any of the directories in your package have
special permissions, then DO NOT reset them here!

Would you like to reset all directory permissions to 755 (drwxr-xr-x) and
directory ownerships to root.root ([y]es, [n]o)? n

Creating Slackware package:  /tmp/xdm-slackware-theme-2017.1026-noarch-1_SeB.tgz

./
usr/
usr/libexec/
usr/libexec/setxdmbg/
usr/libexec/setxdmbg/setxdmbg.sh
usr/doc/
usr/doc/xdm-slackware-theme-2017.1026/
usr/doc/xdm-slackware-theme-2017.1026/s-logo-on-grey.xpm
usr/doc/xdm-slackware-theme-2017.1026/README.html
usr/doc/xdm-slackware-theme-2017.1026/xdm-slackware-theme.SlackBuild
usr/doc/xdm-slackware-theme-2017.1026/adoc/
usr/doc/xdm-slackware-theme-2017.1026/adoc/setxdmbg.txt
usr/doc/xdm-slackware-theme-2017.1026/adoc/README.txt
usr/doc/xdm-slackware-theme-2017.1026/setxdmbg.html
usr/doc/xdm-slackware-theme-2017.1026/README
usr/doc/xdm-slackware-theme-2017.1026/slackware-s-logo.xcf
usr/man/
usr/man/man1/
usr/man/man1/setxdmbg.1.gz
usr/bin/
install/
install/doinst.sh
install/slack-desc
etc/
etc/X11/
etc/X11/xdm/
etc/X11/xdm/slackware/
etc/X11/xdm/slackware/Xbuttons_bar
etc/X11/xdm/slackware/Xsetup
etc/X11/xdm/slackware/Xsetup.conf.new
etc/X11/xdm/slackware/Xstartup
etc/X11/xdm/slackware/xdm-config
etc/X11/xdm/slackware/pixmaps/
etc/X11/xdm/slackware/pixmaps/C1-Nights-Haze-Purple-2.jpg
etc/X11/xdm/slackware/pixmaps/C1-Nights-Haze-Grey-2.jpg
etc/X11/xdm/slackware/pixmaps/C2-Sunset-Shore-2.jpg
etc/X11/xdm/slackware/pixmaps/C2-Ocean-Side-Sunset-2.jpg
etc/X11/xdm/slackware/pixmaps/C1-Nights-Haze-Blue-2.jpg
etc/X11/xdm/slackware/pixmaps/C2-Green-Moutain-Side-2.jpg
etc/X11/xdm/slackware/pixmaps/C1-Steele-Star-2.jpg
etc/X11/xdm/slackware/pixmaps/C1-Shuttleworth-Galaxy-2.jpg
etc/X11/xdm/slackware/pixmaps/s-logo-on-white.xpm
etc/X11/xdm/slackware/pixmaps/C2-Morning-View-2.jpg
etc/X11/xdm/slackware/pixmaps/C2-The-Rivers-Of-Cuomo-2.jpg
etc/X11/xdm/slackware/Xbuttons_bar.conf.new
etc/X11/xdm/slackware/Xresources.new

Slackware package /tmp/xdm-slackware-theme-2017.1026-noarch-1_SeB.tgz created.

LXDE) session=/usr/bin/startlxde;;

LXDE) session=/usr/bin/lxsession;;