You need DTXBmp for textures for FS2004 (DTX3 BMp format)
You need Imagetool the latest version inside the FSX-SDK for FSX textures(DTX5 DDS format)
Photoshop 7 or Paint Shop Pro (but always use it in *.psd the photoshop format)
Grab any of the images of FS9 the one that carry the fuse paintings and drop it in DTXBmp software and save it as bmp 24 bit image for you to work ,after you finish the repaint re-export again to dtx3 (Make sure you mad a back up of its alpha chanel
Grab any DDS images and drop it in DTXBmp software FLIP the image
and save it as 24 Bmp images ,after you finish export by using imagetool latest version in FSX-SDK
DTXBmp its a freeware I'mk Not sure if this the latest version YOU NEED THE LATES VERSION OF DTXBMP IF IS NOT GOOGLE IT ..YOU WILL FIND IT
http://www.simviation.com/fsdesign_utilities.htm