> For example: the site may have references to 
> tn_filename.jpg. I would like to have HTTrack remove 
> the "tn_" and fetch the resulting file (filename.jpg) in 
> addition to or INSTEAD of the tn_ file.

This isn't possible (actually this is possible - but it 
involves some C code and compiling skills) ; the file 
structure can not be "fully" customized.