Sometimes we have to make our own image for GRUB splash background. But there are some restrictions on that image for GRUB bootloader to accept and display it. By ImageMagick, we have an easy way to convert any image to satisfy that:
convert oldsplash.jpg -resample 72 -sampling-factor 2x2 -resize 640x480\! newsplash.jpg
Hint: The \! means FORCE, so it maybe breaks original ratio.