Changing or setting an image to GRUB is trivial. For it we need install two tools of our linux environment:
convert Useful tool for image process (format, size, etc.).
startupmanager Graphical Grub manager.
Selecting the image
Look for the image that you want to be the GRUB background. Is not necessary search for a specific format, size or color deep.
Convert the image into a new GRUB compatible one
The images that can be GRUB backgrounds must have a concrete format, size and color deep. For transforming the image to a new compatible one, you can use the convert tool as:
convert image.jpg -resize 640x480! -colors 14 -depth 8 image.xpm.gz
Configure GRUB with the new image
Once we have the compatible image, we have to set it as the default GRUB background with