Placing a feature centrally in frame Bottom

I think this might be of more general interest than the specific Brass Plate frame. (node/45681) So I am copying it into a new thread here.

What we really need is to be able to change the way the bottom of the frame is made, so that we have another image BBC.gif which is automatically placed in the centre of the bottom of the frame.

because clearly the way frames work is that the BB.gif repeats across the whole bottom, with the BL.gif, BBL.gif, BBR.gif, and BR.gif layered above it in their own positions.

BL and BBL must be aligned left, and BBR and BR aligned right.
Surely it would not be difficult to put BBC.gif aligned centrally, but I don;t know how to do that.

It would also be useful to be able to put a feature centrally on the top and sides for making the old style shaped frames. That would need images called TTC.gif, LLC.gif, RRC.gif as well as BBC.gif.

Gaynor