You will need some type of photoshop program (Paint Shop Pro, GIMP, Photoshop), something that can save Alpha Channels since part of the image is made to be transparent.
I would post a link to a tutorial, but I cant seem to find it. But I am sure if you google using the name of whatever photo program you have, you will be able to find a dozen or so references on how to save alpha channel tga files. The main thing is use the original art to go by when making new art till you get the hang of it or develop some templates.
Here are the textures that I made for XArt -
http://donkies.wildcalm.net/Textures.zip - No need to rename or change anything, just replace the currect textures you have now (after backing them up if you decide you dont like mine).