Its pretty much about finding an image with the correct lighting for the snow.After that its a combination of cloning and distorting, copying parts around the image and blending them together.But if you dont have the correct lighting it will become very obvious very quickly.
What you need to do is find images that have the same colour sky as the sky in the render.This way,not only will you have the correct lighting,but also the correct radiosity colour on your inserted elements.