Looked at your bmp and everything appeared good to me right except it lacked an alpha channel on the second one.
I tried to do a step by step explication for your second image to work. I downloaded the latest Gimp version on Windows to be sure using the same name for the menu (the ubuntu 12.04 version I use is slightly outdated) and.... just discovered the same issue as you.
Crawling on the web, found a lot of complaints with the latest versions of Gimp and BMP. Even with the new "compatibility mode" checkbox it didn't worked .
So I will have to look in it what they did with the file format on the latest Gimp versions.
For interim solution until we found a way to fix it would be to get the 2.6.12 version of Gimp. I just tested it OK on Windows :
http://sourceforge.net/projects/gimp-win/files/GIMP%20%2B%20GTK%2B%20%28stable%20release%29/GIMP%202.6.12%20%2B%20GTK%2B%202.16.6/All images must have an alpha channel (Layer -> Transparency -> Add Alpha channel), event the images that don't use transparency. And the inventory image should be done (use the same as the center). For now, it isn't used but will be in the future.
For images using transparency like glass, it must have a voxelinfo. It's not needed for regular voxels.
The Blackvoxel Team