The random green splotch needs to go. Pretty good for a first try Colors, aside from the green, match up with the render. For next time try doing effects over parts of the render to make it look like it wasn't just a last second thing that you pased in at the very end.
Here's a good rule of thumb, if at the end you can see the render and think, " i could switch that out with any render right now and it'd look pretty much the same," then you need to blend more.