If you don't care for the default Grub wallpaper in openSUSE, this page will help you change it!
The WakeUp.jpg image that I mention in my instructions below was 800x600 but it didn't display properly at bootup. Originally it had too high of a color depth, or maybe the file size was just too large (I don't know because it's almost impossible to find details or specifications for Grub for openSUSE, and every site says something different) so I opened it in Gimp and saved it as a new file, but then moved the detail slider down to 50%. This made the image a little less detailed and it also reduced the size from 600kb to maybe 75kb. After I did this, Grub displayed the image perfectly.
Mileage may vary.
Here are the steps I used to update the /boot/message file (type: cpio) with my new wallpaper:
cp /boot/message .
cpio -idv < message
mv message ../message-old
cp ~/Documents/wallpaper/WakeUp.jpg back.jpg
ls | cpio -ov > ../message
cp message /boot/message
Reboot the computer to test your new Grub Wallpaper!