Using the swapDepths method

I have a small photo gallery that shuffles the thumbnails randomly. When you click them, they move to the center, which I’ve figured out using the slideTo method.

The problem is that when I use:
this.swapDepths(1000);
to bring the image to the front, I can’t figure out how to make it go back to a lower depth. I’ve tried using (0), (1), (-1000), (back), (bottom) and a whole slew of others.

I’m positive there’s a way to send something to a lower depth, but I can’t figure out what it is. Anybody have a suggestion or run into this problem before? Any help you could give me would be great. Thanks.