It's called dds not dss
http://en.wikipedia.org/wiki/DirectDraw_Surface
Edit it with
www.gimp.org (it's free - even open source) with the dds-plugin installed
http://nifelheim.dyndns.org/~cocidius/dds/
This paint thingy might be alright to edit small, uncomplicated stuff like the umark.dds but for real texture work use a real graphics program like the above
