BlueCurls grub2 background. Copy the file BlueCurls.png to /boot/grub/BlueCurls.png. Add the following lines to /boot/grub/grub.cfg (to the preamble before the menu entries):
[quote][indent]insmod png
background_image /boot/grub/BlueCurls.png[/indent][/quote]
. You may also need the lines:
[quote][indent]insmod vbe
insmod gfxterm
set gfxmode="1280x800;1024x768;800x600"
terminal_output gfxterm[/indent][/quote]
. (These lines should appear before the lines for background.) The image is meant for screens of aspect ratio 16:10 or 16:9. For a screen of aspect ratio 4:3, use the image BlueCurls43.png.
For other Linux distributions or to use grub-mkconfig, edit the files in /etc/grub.d/ instead of editing grub.cfg directly.

Also see BlueCurls fbcondecor(gensplash), kdm, ksplash and wallpapers.