You can open Texview's example targa image in GIMP and examine it there, what's different?
Then try to change it and open in Texview.
#EDIT 1:There
might be a bug in GIMP's tga plug-in which affects how alpha channel is marked to the saved tga file. Investigations continue...
#EDIT 2:...and the solution for the problem is to uncheck the "RLE compression" checkbox when saving a TGA file in GIMP. Then Texview is able to open it.
#EDIT 3:"RLE compression", not "RLE packing"...
#EDIT 4: If you are saving an old targa image with GIMP, use the "Save as..." instead of "Save" and you'll get the popup box where you can disable "RLE compression". With "Save" it doesn't appear if it's an old file.