I opened your image in photoshop, selected pixels, created work path, applied a layer and vector mask, then resized it with the ration fixed. This left an image with 1680x1264. I removed the layer and vector mask, deleted the work path, the using the rectangle select to mark the upper half of the image above the sunset. Then using the select tool with the box at top checked for 'show transformation controls' selected, grabbed the box at top and drug it down approx. 180px. Then selected the image only and cropped off the clear space left at the top of the canvas. I got lucky and it came out at 1680x1046, so a simple resize later this is your 1680x1050.