dds texture are images files (I believe gimp can open them)
Maps are in binaries/data/mods/public/maps/
https://trac.wildfiregames.com/browser/ps/trunk/binaries/data/mods/public/maps
There are three types of maps, Random (Procedurally generated using javascript), Skirmishes, and Scenarios. The last too are mostly the same apart from the fact they are in a different folder. Some maps have trigger scripts, which are scripts used to generate specific actions (eg spawning units regularly, or adding a different victory condition)
"Normal" maps use an XML file and a PMP file (The xml contains map settings, lighting, water height, entity positions). The pmp contains height data, and texture data.
wiki:PMP_File_Format