Well you can't create PNG images with free form outline I'm afraid (if I'm understanding you correctly, you're talking about ignoring the transparant parts of a png right?) so that won't be possible, the drop shadow would just wrap around the actual outlines. Although it is possible to apply a border radius to your png and add a drop shadow to those, so rounded corners or circle pictures should work.

But yes, if you have actual free form outlines you would want to shade you'll have to resort to adding them to the PNG, but for most elements we can use CSS.