How to add a shadow effect to a PNG image???

Hi, I just finished editing this pic, and I don't how to add a shadow effect to it....

I tried it via Layer-Layer Style-Drop shadow(CS4), but those options are unavailable when I select the file...


What I'm trying to accomplish is the same effect like on this pic here:






This is the picture I want with the shadow effect:





I would be grateful if someone could do this for me, and/or explain me how...

the image takes up all the canvas resize the canvas and apply your effects or resize the image and apply

btw probably outer glow effect would work best