By rendered image I take it you mean rendered in another programme,such as poser or bryce. Both of these programmes support alpha channels in tiff or photoshop formats. You do as your doing and load the Alpha channel as a selection,then go to the quick mask button and click on quickmask.If you double click the quickmask icon you should get an options pallette that allows you to set the mask as covering selected or none selected areas. If you want to refine the mask you now need to do some panting around areas like hair in close up.Once your happy with the painted quickmask,turn it back into a selection.If your main figure is selected I find it best to copy then paste the selection onto a new layer,then I,m free to do what I want with the background.If not invert the selection. Varying opacity is easily handled by duplicating the image on another layer.Removing the semitransparent section from the top layer allowing the duplicate to show through and altering the opacity of the duplicate layer to allow some background to show through.