I have an image I'm using as background which is 3000px by 1500px. For responsive design, I need to extend it to 3000 x 3500. It is an image of a river, and I want to continue the look of the river as seamless as is possible. I've tried copy and re-pasting parts of it but you can see the repeating pattern. I've also tried the Clone Stamp tool, but that didn't help much.