Grub splash image requirements:
1. xpm.gz file type
2. 640×480
3. 14 colors only

You can use GIMP or just simple convert if image simple.

convert -resize 640×480 -colors 14 orginal.xpm splash.xpm -normalize -verbose -depth 14
gzip splash.xpm

vi /boot/grub/grub.conf
splashimage(hd0,1)/grub/splash.xpm.gz

nice red splash: red_splash.xpm.gz

sexy gril splash: girl_splash.xpm.gz

Hints for Gimp:
Image -> Scale Image
Image -> Canvas Size
Layer -> Layer to Image Size

Filters -> Blur -> Selective Gaussian Blur
Image -> Flatten Image
Colors -> Maps -> Gradient Map

Colors -> Levels (auto)
Colors -> Posterize (14)
Image -> Mode -> Indexed (14)

and etc. 🙂