Sketch Picture & Alpha Layer

I often work with sketch pictures from industrial designers to create solidworks geometry. Unfortunately the sketch backround interferes with viewing parts in assemblies, especially during rotation. So the simple solution is to open the image in GIMP, photoshop etc and convert the background to an alpha channel. I am using the PNG filetype. When you bring this back into SW as a sketch picture, the alpha channel is not recognized.

Does anyone know a workaround?

SolidworksSketching