This is my first contribution

You can use diffrent backgrounds included in this pack.

After install in /usr/share/gdm/themes/, just open /usr/share/gdm/themes/haveaniceday/haveaniceday.xml,

and change :
"grass2.jpg"

to :
"bee.jpg"
"church.jpg"
"grass.jpg"
"riverraid.jpg"
"chessboard.jpg"
"grass2.jpg"
"meadow.jpg"