The title of the thread is 'Art of the Thread', being pretty clearly a reference to 'Art of the Dress' or Suited for Success, or whatever. And while Rainbow Dash makes everything 20% cooler, when it comes to sewing based themes, there are really only two ponies up to the task: Fluttershy and Rarity (darling!).
Trixie's known for changing the image in the OP on a fairly frequent basis. You might see cameos from others, but I have a feeling two ponies in particular are going to wind up dominating our attentions...