How to take pictures to lingerie on a mannequin but without having the mannequin visible in the pic?

I assume you mean that you want the Camera inside of Unity to ignore the mannequin 3D model?

If so, set the mannequin to its own Layer and on the Camera settings, disable that Layer in the "Culling Mask" option. The Camera will render everything except that layer.

If what you want is to edit a real life photo to remove the mannequin in Photoshop, you will get better answers on forums that deal specifically with Photoshop. There are also tutorials on how to do this with glass instead of fabric. Perhaps that could help.