The best way to edit these source files is using photoshop

You may also try using the free graphic software called: GIMP

http://www.gimp.org/

I personally don't use GIMP, but I know you can use GIMP to 
edit photoshop files. But, GIMP is limited and you may not
be able to edit every element.

So, it's best to use photoshop if you can. One way to get
a cheaper version is to find an older version of photoshop
on ebay.