Its not a quick solution but AutoCAD 3D and other 3D software will allow you to add lighting schemes to your drawings. This requires you to draw in 3D though. I doubt you can just scan a picture and get a 3D effect.

Programs such as Paint Shop Pro and Adobe Photo Shop allow you to create overlays which you could use to overlay a light fixture.

Perhaps if you used a image of a light fixture against a solid background you would be able to cut out the light pattern and overlay it transparently over another image. This would atleast give you an idea of what it would look like.