If u save .tga files in photoshop jsut save it as you would save any other filtetype. When that small window pops up choose 32bit/pxl and uncheck the "RLE compression", then it should work.
If u got a transparent texture use alpha channels to get the transparency working (as far as u dont know what alpha channels are, google for tuts).
Alternative way for convertign transparent textures is savign them as .png, open them with gimp and save them as .tga there, GIMP will handel the transparency by its own.
For previewing textures.
-go to C:\\programs\Toribas x.xx\custom.
-create a folder called "test" there.
-put your tga named as "head.tga" in there.
-start Toribash
-type /lp 0 test
-enjoy the preview
For your customers to uplaod the textures:
Send them the Images as 128x128 pxl .png files, then they only need to upload 'em.