I was not happy with the default background colours of GRUB.
Long story short.(I am on KDE Plasma, maybe will work with other DE)
-Download or copy whatever image you like.
-Open the image
-Save it as .png (save as not just rename, I tried and it detected it is not really .png)
Then
sudo cp /home/limo/Pictures/coralsoceanunderwater.png /boot/grub/themes/EndeavourOS/background.png
You may like to backup the original or default “background.png” in case you need to return back!
I did it and it is working with me fine.
Rebooted a few times with the image I selected.
P.S. My first photo was a very light top left corner so it was hard to read the menu. I selected another photo with a dark top left area so menu is easily readable. (don’t know how to change font or font colour, yet!)
I hope you like this one. 