Have u tried
1. Extracting the Rez File using RezExtract
2. Using DTXConvert to convert your DTX file to lithtec
3. googleing plug ins for gimp so that you can use .Dtx's in gimp
4. then opening up the Dtx in the gimp
5. making changes.
6. save the Dtx
7. Convert Back to Nexon
8. Inject and be happy.