Quote: "If ?Make@@YAPAUIDirect3DTexture9@@HHH@Z takes 3 parameters (ImageID, Width, Height), how does it determine format (bit-depth)?"
By default it uses the same depth as your backbuffer.
Quote: "Do some of those functions directly reflect the data that's in the D3DSURFACE_DESC structure? Like 3D3FORMAT format and DWORD Usage"
Yes.
I think the best thing for you to do is to read the code for the plug-in itself so you can see:
http://code.google.com/p/darkbasicpro/source/browse/trunk/%20darkbasicpro%20--username%20LeeBamberTGC/Dark%20Basic%20Pro%20SDK/Shared/Image/CImageC.cpp [edit - sorry, it won't link correctly - just copy/paste]
IIRC, all the functions you've named effectively call the MakeFormat function, so there's a good place to start.