Picking 2D Graphics from Canvas

Hi all,

i have a question. Ive made an isometric drawing "engine" and with the mouse you can "select" these iso fields. But now i need something more complicated:
I want to pick graphics directly, the problem is that: Imagine a table. On the table is a plate, with an apple on it. If you hover with your mouse-cursor over the apple the grid selection is actually behind the table, so you will never be able to pick it up. It looks wrong.

Does someone have an idea how i could find out which image the mouse is over?
All the images are objects. So clicking on the apple should sooner or later return the object.

Thank you!