Perhaps I can be of assistance. I use Gimp, and I've done some texturing.
One thing that might have happened, if you were not careful, is that you may have saved your work from GIMP in native GIMP format, which has the file extension, ~.xcf. I find it useful to always use "save as" instead of "save", as this always brings up the dialog box, and lets me verify the file format (as indicated by the file extension), and the file name. And if saving as a ~.tga file, after the save indication, there is another dialog box asking whether the file should be compressed. In this box, the selection box besides "BLE compression" should NOT be checked.
Another issue, if you change the size of the image file, you may have changed it to a size that Trainz does not permit. In Trainz, the image sizes are a power of 2, that is the dimensions of both sides are 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, or 2048. While you can save rectangular images, e.g. 64 wide by 512 high, you want the image as close to square as possible. So 64 x 512 is to be preferred over 16 x 1024. I have read, but cannot personally confirm, that the shorter dimension of a rectanlge should be less than the sixth higher value in the above sequence, so that 2 x 64 would work, but 2 x 512 would not.
The error message you posted,
C:\Program Files (x86)\Auran\TRS2006/local/content hgxuxthe299/mesh_body/Nose.tga
might result if you erased the "Nose.tga" file, if it is compressed, or if it is no longer the proper proportions.
ns