MW's DXTBmp should work with any graphics editor. It's basically a texture converter that will open & save image files in 24 bit & Extended BMP formats. I once tried it with PhotoPlus 5 which does have a few peculiarities. If I remember correctly you might have to save in BMP format using the File/Export option. Then load the saved BMP separately into DXTBmp to convert it back to an FS texture format. This should not be a problem once you get used to it.

The only alternative I know is Imagetool. This is an M$ utility included with FS2002 Pro. I believe it might also be included with the FS2000 Scenery SDK & the FS2002 Aircraft Design SDK posted on the M$ site. The FS2002 version is later with more options. It would also have to be used with PhotoPlus 5 or any other graphics editor in the same way I described earlier.

Imagetool is a little standalone utility. It can't be linked to your graphics editor like DXTBmp. You would have to use the method I described to use it anyway.

Here's the FS2002 Insider download page. http://www.microsoft.com/games/flightsimulator/fs2002_downloads_sdk.asp
I'm not sure which SDK includes Imagetool. I think it might be the MakeMDL SDK.

PS. I might be completely wrong about Imagetool in those FS2002 SDKs. Mine came with FS2002 Pro. I never use it.