So far, from what I've seen, they appear to be in the asset.dat file. I did a bit of digging in the command button and mapped images files, and found a file named in the mapped images to be located within the asset.dat file. I can't see how the files are actually stored there, as there is not a lot of stuff other than file names in asset.dat, so I'm guessing it is an index file for another file, providing binary offsets. Perhaps it's all stored in langdata.dat? That appears to be nothing but binary/non readable information. I might see if I can find an image file from asset.dat by removing a section of langdata.dat.
