So, you want to change your Grub background or wallpaper?
First, find an image that is at least 640×480. Open the image in GIMP.
Now, resize the image to 640×480 if needed ( Image -> Scale Image… )
And then save the image as an XPM ( File -> Save As… ).
Be sure to save the file as an X PixMap image (.xpm).
Compress the file and place it on your /boot partition, inside the grub folder.
gzip grub_background.xpm sudo cp grub_background.xpm.gz /boot/grub/
Finally, edit your /boot/grub/grub.conf and add or replace the splashimage entry with your custom background.
Reboot to test your changes. If the Grub background appears like television snow, check that the image size is exactly 640×480.