I use gimp as my main graphic editor. The fastest way is download dds converter 2.1. In second can convert any image, or folder to your format. Generally what I do is save file as psd, and then I convert it via this program to tga.
So now I have them in the exported format, how do I change them from a folder back to the BSP file? unless im missing something and im supposed to use the Export1 button but that gives me "Wrong model to export" error