(Sorry if this is the wrong section, I think I never posted in the Computer section before so I don't know at all how it is organized).

So I have a dual-boot on my PC, and GRUB2 between the two OSes.
I found GRUB2 quite ugly so I wanted to customize it a bit. I downloaded a theme for GRUB2, applied it, and found it ugly too. So I thought about modifying the components of that theme.

So it had a background.png image. "Self-explanatory name" I thought. So I basically replaced it with another png image I made with Paint.Net with the same dimensions and did "sudo update-grub". I rebooted, waiting for my new image to show up and... an error comes instead, saying something like "wrong format".
I thought the problem could come from Paint.Net so I did the same with a png image from Gimp. Still the same error.
I thought that the initial png maybe was special so I tried a jpg from Gimp. Another error (don't recall it exactly).
For some reason I tried with a jpg image from Paint.Net. It worked.

So dafuq is happening ? Is there format differences within the same extension (I can think of compression but what more ?) ? Which extension+format should I use to produce GRUB2-compatible images (Paint.Net jpg works, but has no transparency) ? In other words, which software should I use to produce a GRUB2-compatible png ?

I know you have almost complete control of an image if you use Imagemagick's convert tool.