Without going into the rights and wrongs as to whether a space ship should be default theme for Debian Squeeze I for one needed to change it. The only thing I want to say on the matter is that then Debian Lenny was the first and only default Linux theme I have kept. I loved it completely, it was clean, fresh and sexy!
So for a temporary work around I was using the Flow wall paper, the only thing I did not like about it is that there was no beloved Debian symbol.
Step 1: Create my media:
Of course you can use a range of formats and scales but here is what worked best for me. (BTW you can not upload tga files which is required for Grub; simply save via GIMP as a TGA file.)
For resizing I have a nifty nautilus extension application which adds a resize and rotate option into the context menu.
The flow image has the background as transparency. I was not convinced that GRUB 2 or GDM 3 would support transparency or whether it was possible to configure a background colour within them. I decided it was easier to work without transparency. The default background colour for Debian (or there abouts ) which I had become accustomed to is: Hue: 20; Sat: 30; Value: 58 or R 104; G 128; B 147.
I added a Debian logo, Debian text (from logo)and played around with the drop shadow plug-in, transparency, colour etc and came up with the following design.
This suited as my background paper. GDM 3 and Grub 2 require the following formats:
GDM 3: PNG image with dimension of 1024×640
Grub 2: TGA image 640 x 400
Their location is not of to much importance but I placed mine in /usr/share/images/.
It might be worthwhile here to note that it might be good to make backup copies of any files before you edit them.
Step 2: Configure GDM 3
This is simply achieved by editing the file
/usr/share/gdm/greeter-config/10_desktopbase.
Add in your picture here.
/desktop/gnome/background/picture_filename /usr/share/images/DebianFlowLogin.png
Step 3: Configure Grub 3
This is achieved by editing the following file:
/usr/share/desktop-base/grub_background.sh
Add in your picture here:
WALLPAPER=”/usr/share/images/DebianFlowGrub.tga”
Update Grub:
update-grub
That is pretty much it.
I could not upload a tga file to wordpress but here are the login and wallpaper screens.
luke wickstead 
And why you do not try update-alternatives –config desktop-grub ?
of yourse, first install new alternative – IMHO it is better than change files from any package 🙂
Its just the way I did it.
you can also add your own file with a higher xx_; this will get loaded later and override any changes before.
thank you! worked great