I'm not very good with any painting program but recently I tried the following.

Load the dress into DS then export it as an OBJ into a temp folder.

Open up the dress OBJ into "Sculptris". It is a free 3D modeler from ZBrush.
http://www.zbrushcentral.com/showthread.php?t=090617

Sculptris will recognize that the OBJ already has a UV map and then asks you if you want to go paint it or sculpt it. Answer that you want to paint it and you are taken to it's simple paint room.

In Sculptris paint room you can do your painting on the 3D model. maybe can use it to paint simple guide lines or rough work then you can export your painted UV map from Sculptris and open it in Gimp or Photoshop to do further more detailed painting.

Note Sculptris may crash depending on how large the OBJ is and what your PC system is.

Following is link to video tutorial for painting in Sculptris.
http://www.zbrushcentral.com/showthread.php?t=91753