Combining grids with layers usually leads to headache, but you can make the layer center with the rest of the content if you put it inside a container div that's the same size as the grid, and let its position be relative to that.
Here's a tutorial that should clarify:
<http://mindpalette.com/tutorials/center_layers/index.php>
You'll also need to check that the layer's z-index (and the container's z-index) are higher than the rest of the content - noticed that in Explorer 7 it ends up behind the grid content.
Or (better) add the image directly to the grid and skip the layer.