
and who told you that dtxbmp is for working in dds format?
neither photoshop although you can open things with it
You have to work under PSD images Then Once you complete the repaint You will drop the bitmap.PSD into Imagetool the it will flip it for you you can compress it into ddsdtx3 or ddsdtx 5 and save it JUST for the use of FSX game
The official software to compress DDS. Open, edit etc is ImageTool.exe is located in the FSX-SDK
In WinXp
C:\Program files\Microsoft Games\Microsoft Flight Simulator X SDK\SDK\Environment Kit\Terrain SDK\Imagetool.exe