Comment 5 for bug 1525687

Okay. After spending a lot of time on this issue I finally found the answer. I want to share.

The reason why certain images don't work is because they do not meet certain requirements. To find out what those requirements are and how to make an image conform please read this...

https://help.ubuntu.com/community/Grub2/Displays#Choosing_a_GRUB_2_Background_Image

There is also one additional requirement I discovered on my own, i.e. the path to the image must not have any spaces. For example, ~/Pictures/my image.png will not work. Instead, try ~/Pictures/my_image.png.