(click image for larger view)

Quote - But rendering the image the parts of her below the ground are also rendered. is there a trick i can avoid that?

There are really two things that need to be done here, 1) make the ground visible and opaque, and 2) project the background picture into the ground plain in such a way that the perspective looks correct from the current camera angle.

Here is how I made the above image. Obviously, if the ground plain is to hide anything, it needs to be both visible and opaque. Make sure it is turned on; from the menu bar, Display > Guides > Ground Plain. Next you needed to make sure it is opaque, the most common reason for it not to be opaque is that "Shadow_Catch_Only" is turned on, you need to turn this off in the ground plains materials (see graphic above). Whilst you are in the material room, set the Diffuse_Value and Specular_Value to zero, set the Ambient_Color white, and the Ambient value to one. Set the picture that you are using as the background to be the Image_Source for the ground plain, don't worry if it looks wrong at this stage, we will fix that in a moment. Plug the image into the Ambient_Color node.

Lastly we need to adjust the perspective of the image on the ground plain. In the Pose Room, select the ground plain, open the Group Editor. Create a New Group and Add All, then "Create Perspective UV's".

This seems to work for me, as evidenced by the above image. I can't guarantee this will work for you, there may be details I have left out, for instance the size of the background image may need to match the size of the final render. I have only done this a couple of times myself, so I'm still learning the technique.
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Of course the Perspective UV's will only be correct when the camera is in the same position as when the UV's were set, so you need to decide on the camera position for the final render before you set the Perspective UV's.