Generally speaking, tga's that are packed in dat files must be located there for SH3 to detect them... for instance, the U-Boat HULL skins... However, you can put the DECK skin files in tnormal/tex and things run fine.
I hope that helps... and if I am incorrect I am sure someone will correct me.
Now.. WHY things are the one way for some tga's and not for others is really beyond me for the time being.