如何在longlistselector中设置datatemplate的zindex

I have a longlistselector with certain images that i add in this longlistselector. I also change the margins of this image to make the image go up or down and so on. But i want to put this image infront of another image in this longlistselector. I have tried using Canvas.Zindex. I have tried setting it at the grid level, at image level and at the top level of of the longlistselector.() But it still doesn't work. Does anybody have some idea's? You can find my code bellow:I have a longlistselector with certain images t