I installed GC and the grub2-splashimages package. My grub.cfg file looks the same as yours except I see you are still using a jpg image - I assume this is yours unless you converted one of the downloaded images. The package files are all .tga files.
If the one you are using is not one of the downloaded images, make sure the image is RGB (you can tell in Gimp via Image > Mode. Make sure it's RGB and not Indexed).
If this doesn't solve things, and you haven't tried using one of the /usr/share/images/grub tga files, try one of those.
The only other difference I see is your resolution is set to a specific value while mine is set to auto. You could try manually editing /boot/grub/grub.cfg and changing the gfxmode setting to "set gfxmode=auto". If you try this, do not update-grub before rebooting.
Edit: Looks like you edited your post as well and have already tried a .tga file...
One other thing that may have happened. Grub 1.99 (Natty) uses the first image file it finds in /boot/grub if you haven't specifically designated it in /etc/default/grub with the GRUB_BACKGROUND setting. When you set the image in GC and 'save' it should use the correct file, but to be absolutely sure I'd remove any other images from /boot/grub and leave only the one you really want to use.
I've just confirmed my suspicions in Edit 2. Even though I selected the tga file I placed in /boot/grub, with an old .png file in the same folder, grub.cfg used the .png file instead. So if your .jpg file is bad, Grub2 may not have used the .tga file. I'd remove your jpg file and select the tga file once more in Grub Customizer. Don't forget to press SAVE.