-Snow Covered Landscape- (In Full Colours Only)

It is worth thinking in advance, especially where there are obvious vertical objects near the edge of the composition, and leaving a little extra room to be able to do corrections later. I guess if you were on 16 mm you'd not got much choice here. One thing to consider is increasing the size of the canvass to leave empty space around your image then selecting the canvass as well as the image and correcting it all. That can allow you to keep the edges of the top of the image and you can try to fill in the empty triangular spaces left on each side using cloned parts of the image from nearby either using a progamme like Snapheal or manually. It is going to be hard on this picture with these nearby small trees but a more plain surround can often be successfully cloned and fool the eye of the viewer so it is not noticed. I think I'd be happy to lose the small trees entirely by removing with Snapheal (or Lightroom equivalent) or cloning manually.