The new version of the AOM Data File Converter (or AOMEd etc), downloadable
here (needing perl56.dll unzipped into the same location if you don't have it already) can now convert DDTs from textures.bar into BMP images. Run the program, click "Direct File Conversion", select a DDT, select a new BMP, and it should create the bitmap.Some DDTs include an alpha channel, making certain bits of the image transparent; but it's not always used, so you need to edit the 'invisible' bits, so it's not much good to convert it into a TGA/etc with alpha since it'll get all confusing... So, the BMP often contains a black-and-white image below the colour image, which is the alpha.
It's not yet possible to convert back into DDTs (since it's rather complicated ) but AOM will apparently sometimes read uncompressed TGA files from the "textures" directory. But if that doesn't work, you can still look at the pretty pictures