DXTBmp is a program used to open and convert various image types from and to .dds files. You can not edit the skin in DXTBmp, you have to load the skin into another program, e.g. Photoshop for this. An image editor of your choise can be chosen by clicking the icon on the right, which will allow you to load the .exe of the image editor.
DXTBmp was developed by Martin Wright. Official Website