Generally speaking you're going to need access to some form of photo-editor like
Photopaint,
GIMP,
Photoshop or any one of the available applications that can be had for free or purchase.
It should then be a few simple steps to save textures as TGA files. First the original image/s will need to be opened into the photo-editor, so depending on the format that should be as simple as "
File >> Open" or "
File >> Import" - obscure or unsupported formats may mean switching to a different image editor from where it should be possible to export the images to TGA without then needing to then re-open the converted images back into the application you were originally going to use.
Either way, once the image is in the application you're using, it should then be another series of simple steps; "
File >> Save As" or "
File >> Export", selecting the
*.tga (TARGA) image format from the available options (a list of available formats is usually displayed as part of the file saving process); although again, subject to availability.
