It looks like you rendered the background separately without the figures, as I see no shadows. What you could do is assign a gray shader to the characters, render it with shadows so they cast them, then render the characters separate and layer them over the original image. Be sure to use some kind of matching colour to the background (grayish green, it looks like) as a background colour when you render the characters so the edges aren't so pronounced on the alpha channel.
One other thing, if you can either adjust the dress so it bends better... perhaps there's a bone that'll allow you to do that, or adjust the weight map, or use a dformer on it... I know that's quite a bit of work...
Actually it is one and the same render. The background is with multiplane cyklorama. I know it isn't realistic or any good for doing much but in this concept I thought it worked quite well. There is in fact shadows but very faint (tends to be so with Uberlightning). The unicorns right foot(shoe? paw?) doesn't cast shadow because it is slightly stuck in the ground and then DAZ won't recognize it for raytrace. Don't know why and I just saw it.
The dress sucks because when one object that belongs to another model is fitted to Genesis it looses much of its "bendability". I haven't gotten around to reinstall K4 yet but when I do I'm pondering re-render this scene.