Trouble with resize ratio of XAML element

Hi,

I'm currently working with a XAML window-like element which has a
height/width ratio of 3:2 Within this, I display a perfectly square image
which stretches to its grid uniformly, thereby maintaining its 1:1 ratio. My
problem lies with a background grid (being used for illumination effects)
which lies underneath the image. The background grid is a 3 x 3 grid - with
the center section being a transparent empty section where my image fits
neatly over. The other 8 parts of my grid light up when the image is placed
over the center part. When resizing my parent window-esque class, I cannot
get my center piece of the background grid to resize at a 1:1 ratio, because
it is instead based on the parent element which has the 3:2 ratio. The
result is the background grid grows faster than the image itself. Is there
anyway to change the resize ratio to 1:1?

Thanks in advance to anyone who can help.

Regards,
Mike