Your render is just sitting on top of the background. A simple way to blend it in is to hold ctrl and click on the render layer (this selects the render). Then go to the Select Menu and click Inverse. Then right click on the render and choose feather at 7 pix and click Ok. Next hit delete once then hold ctrl and hit 'd' to deselect and your done.
That's something basic but all you really need to do is just go to deviantart and look up some sig tutorials and you'll be making decent sigs in no time.