as said PS is one of the best for the job. it is commercial program though.
I am not sure but you might want to see if Gimp or Paint.net have TGA support so you can edit alpha layers.
http://www.gimp.org/
http://www.paint.net/
For XML editing Notepad++ is a powerful text editor that has color coding build into it (very useful when coding).
http://notepad-plus.sourceforge.net