A way to random background images in GDM3 - Debian 6. Should be applicable to other distributions, the only difference being configuration file paths.
If you know of a simpler approach please let us know.
PRECAUTION: I'm just a newbie customizing a family laptop, this may not work, or slow down or expose your system in ways i do not understand.
Follow procedure at your own risk.
1- Creating PostLogin bash script. #as root
cp /etc/gdm3/PostLogin/Default.sample /etc/gdm3/PostLogin/Default
2- Editing PostLogin bash script, this will take a random image from directory and make it the background image #as root
Add the following to /etc/gdm3/PostLogin/Default and adjust the path to point your images directory.
#change background image
file=`/bin/ls -1 "$dir" | sort --random-sort | head -1`
path=`readlink -f "$dir/$file"` # Converts to full path
cp $path /wherever/you/want/background.jpg
3- Create the background image and change permissions for all users to change background image when logging out.
chmod 777 /wherever/you/want/background.jpg #do not know if this is necessary
4- Editing GDM3 configuration file #as root
Uncomment the following line and change background image path
And adjust config file to your likings.
I reused the bash code in step 2 from somewhere i cannot remember, if you are the author please let me know and i will
link to source.