MS Paint does not support tga files. Download Paint.net http://www.eecs.wsu.edu/paint.net/download.htm it is free.
To edit tga open skin in SkinStudio, first step, click tools, preferences, editing, image editors, check covert tgas to pngs before using image editors. SkinStudio converts png to tga when saved. Then double click on preview the part you want to edit and it will open in paint.net.
