How to reproduce:
- install 12.04
sudo apt-get install grub2-splashimages
- navigate Nautilus to
- observe that only two of the TGA files have thumbnails.
- install Grub Customizer and use Appearance setting -> background image and the two TGA files that have working thumbnails are the only ones that are displayed there as well.
- Eye-of-Gnome (Image Viewer) -> side panel (bottom) -> details -> previous-next arrows only update their thumbnails and other details when the two working TGA images are selected, yet eog can display the TGA files in the main window.
In order to open a bug you'd have know what component has the issue and that's not clear.
Update: if I re-save the TGA files with RLE-1 set they work.
Update: Debian bug 523003 gqview: does not display TGA and PGM files reports this issue. The bug was closed back in 2009 by someone who must have meant well but I tried geeqie and the non RLE TGA files have broken thumbnails in it.
12.10 does not have this bug.