I'm not certain of the stance of editing other peoples works(I would advise crediting the original designer).
That aside, to do it I would save the preview PNG since it's the same resolution as the spray for your base to work from.
I prefer to use GIMP (
http://www.gimp.org/ ) for my picture editing. It's both powerful and legitimately free. Since the PNG has the transparency, all you have to do is make your changes then File -> "Save As" a .TGA file.
Then it's just the simple mater of importing into VTFEdit. I allways forget the settings so I refer back to step 7 and forward on the Create Team Fortress 2 Sprays with Transparency tutorial on this site.
http://www.gamesprays.com/articles/tuto ... parency-1/I'm mostly a spray novice so others here probably have better recommendations but it's how I normally do my sprays.