There's only one way, you have to create a trans map.

Open the image texture of the dress, select the part of the image external at the edges of the dress texture, cancel the selected part or make it black, invert selection and cancel the dress texture or make all white, find the part(s) you don't want to be shown, select and make them black.

Try the texture, load it as texture map in "Diffuse_Color".

When you're satisfied, restore the right map in "Diffuse_Color", load the map in the "Transparency" channel -> New Node -> 2D Textures -> image_map and set the dial to 1.0.

You can also add the same node to the "Transparency_Edge" channel and set it to 1.0.