Well, you've gotten some excellent responses already on this! Here's my small addition. You can imaginatively divide an image into nine equal sections, like "drawing" a tic-tac-toe grid across it. The four points where the vertical and horizontal lines are the most compositionally=balanced areas to place your "center of interest." Note, as has been said already, none of these four points are in the center of the frame. Each one is a bit to the left or right of center, and a bit higher or lower than center. Pick one point and put your main item there. All else should "support" or "balance" it, because that item is what your image is about or for. In your sample image, which I think looks darn good, I'd move the sun to the upper-left point -- that's upper-left point of the imaginary tic-tac-toe grid -- and then I'd move it ever-so-slightly further to the left of the exact point. I think everything else in the image has a good balance and sense of interest. :)