You dont want to put the jpg as the texture... as obviously if you then remove it and just insert the tga i dont think it will work..

Instead in the shader just add the line
qer_editorimage textures/****/*****.jpg where the *** are your directories and name of the jpg and make sure the texture is the tga version.

Then in GTK you can see the texture etc etc...

I still dont understand why you cant ses teh tga though as it shows fine on mine..Have you the latest version or an old one??

Funk