Use a 24-bit color image like this one to create the mask.
Run the Impressionist plugin. Select a Style.
Set Background to Image.
Click on More Controls / Select Color / Set Palette to None.
Move Preview window the show the black/white border.
Some styles will work on the border only.
Other styles will work on the edges and on the black background as well. These can create interesting effects.
See the next image for results.