Here's a great tutorial on outlining images (as opposed to text);

1) Bring in your object
(for this to work properly, it has to be a transparent PNG image for this to work).

2) Duplicate it.
(on the Mac press Cmd + D, on the PC: Ctrl+D ... or, use the MODIFY/Duplicate Element menu item).

3) Click on the SHADOW button (see screenshot below).
Move the Opacity slider all the way down to 0. (this makes the image transparent, but not the shadow).
Move the Shadow Opacity to 100 and the blur radius to 0.

You'll have to scale the original just a bit smaller, but it works! Check out the screen shots below!