I wanted to add something to the default GRUB screen that greets you as you start Linux more nicely than monochrome text.
I did this using the splashscreen feature of GRUB version 1, the boot loader that I had installed.
I used Krita for some of the image manipulation, but GIMP is also a good app. On Debian, you can get GIMP as root with
apt-get install gimp
Splash images in GRUB have to be in .xpm or "X PixMap" format and they have to be 640x480...