Use TGA Flipper, that is what it was designed for. I use it all the time.
TR
TGAFlipper is the best. CTP used that tool sooooo many times that it would be nearly impossible to count. It truly is a life saver. It was our general practice to use it twice, yes I do mean 2 times, on ANY TGA that was exported by nwnexplorer and also any TGA that we were not sure how it was generated. The main reason was that there are more than one definition in the header of a general TGA, and NWN required an earlier version that what the newer tools created. So, TGAFlipper would re-create that necessary header information automatically whenever it processed a TGA file, TGAFlipper also tends to adjust the physical dimensions of the exported TGA files to be proper x2 sizes, IE 32x32, 32x64, 64x64 etc....as odd numbered bits would totally crash the game engine..
SO, I totally agree with Tarot, TGA Flipper, is an absolute lifesaver and an excellent tool to utilize whenever handling TGA files.
Another simple note to folks in general with TGA files, NEVER export from DDS to TGA and then re-compile back to DDS. ALWAYS keep the original DDS, and most preferably, keep the ORIGINAL TGA whenever possible and only generate that final DDS when you are completely done with making possible modifications to the original TGA texture.