Sorry i haven't responed sooner but i have also been trying to figure out this problem.
I can confirm ASRI éducation's findings,
in so much as GIMP 2.8.4 also appears to be 'mangling' jpg images.
![Shocked :shock:](./images/smilies/icon_eek.gif)
I loaded up a copy of back-mike-ok.jpg (after verifying it worked in a GRUB gfxmenu file)
then exported it as a jpg with a different name, at which point it became corrupted
and i could not get it to work in a GRUB gfxmenu file.
This behaviour does not happen when i use GIMP 2.6.10
Another workaround to fixing these 'corrupted images',
is to open them up using mtpaint
and the use Save As to either overwrite it or save it with another name.
That does something which allows the image to display OK in a GRUB gfxmenu file.
CatDude
.