Take a look at the sky, it is darker on the sides and especially in the corners. This is called "Vignetting".
Wiki: "In photography and optics, vignetting ( /vɪnˈjɛtɪŋ/; French: "vignette") is a reduction of an image's brightness or saturation at the periphery compared to the image center. "
You could try to solve this problem by using a Lens Correction filter for example in Photoshop.
But you should trust Dana's words, there seem to be other obstacles to get it accepted.
Anyway good luck and don't give up.
Perhaps take a look at airliners editing guide, it might help: