The ground plane is rather expansive.  An alternate way, use a 1-sided flat plane and match the image's resolution with the x- and y- scale for the prop.  Rotate the prop so it is parallel with the actual ground plane and use the Drop to Ground feature found in the taskbar.  IIRC, it might be under Edit.  Afterwards, you can scale that plane to fit your scene.  And it would be easier to use displacement mapping to get greater detail.