can anyone fix the proview problem?

you can render a cut-out as a mask, and a full render with unreal background. Use the "mask" pic as a mask in Photoshop or something upon the rendered pic. Use the real pic as a background. And fix the edge problem with soft brush and blur tool. Remember to adjust the hue/curve/... to make them meet.

I also found something clean and free in the following place. They are portable.
http://portableapps.com/
http://www.portablefreeware.com/