If you are careful, its fairly straightforward. [In Photoshop] select in your favorite manner the entire black box. [I included the bricks.] For this, I used the "Magic Wand" and the straight line selection tool thingy. Save this selection as an alpha channel as you'll need it. Deselect everything. Duplicate everything to another layer. Adjust curves, lightening everything. The snow will loose detail. Copy this. Delete this layer. Load the selection alpha channel. Paste Into. Flaten, and touch up with the clone tool as needed.