Another note on the TGA2IMG converter. If you're editing a sprite image or texture, it needs to be 8-bit colors. The easy way to do this is to save the image as an 8-bit PNG then resave it as a 32-bit TGA file since the converter errors out at 8-bit TGA files.
There's also a DDS2TEX converter. You can drag all of the DDS files extracted from the TEX file into it, then it prompts you for the original TEX file. Then it will replace all of the textures in the TEX files with your edited ones and produces it as a new file.