As Debbie said, it you're starting with a PNG that has an object on a transparent background, duplicate the graphic layer. You now have Layer A and Layer B. On the Layer B, the lower layer, set the WHITE POINT to -100. Blur to 30% (or 20% to 60%, depends on the desired amount of feathering). Change the the opacity to between 50 and 70. Adjust the Pan-X and Pan-Y to offset the shadow layer from the graphic layer.
Note: changing the black point and brightness to -100 works for many situations but not when the image has lots of pure white (such as a white sky or the wash of a camera flash). In that case, set the contrast to -100. This changes the white to a form of gray ... and gives the other controls something to work with.
There are a variety of techniques to grow or fade the shadow. These involve changing the pan settings, the opacity, and the amount of blur. Increasing blur increases the amount layer the layer edges expand beyond the original size (feathered beyond the original size). Changing the opacity can compensate for the apparent lower intensity as the blur is increased ... Changing the pan settings can give the impression that the light source is changing direction or give the impression (depending on how you do it) that the light source is flickering. You do these changes using a variety of keyframes...
Good luck and have FUN!