First, find the .tga you want to modify and save a copy of it to your desktop or another folder (depending on how orginized you are) and open it with Photoshop. Then when in PS, select the image and go to File/Save as... select JPEG and rename it. Now you have a JPEG. If you are going to do modifications to the files then they need to remain .tga files or they will not work inside the game, and make sure they stay in the .pk3 file format in their proper directory. Hope this helps.