Antonio Lambe

My efforts with Paint.Net, below, to lighten the sky have proved very clumsy and the transition looks so artificial. Any simple pointers for achieving that effect would be much appreciated. Thank you.

Many thanks, Frio, for the prompt and promising reply. But you have generously assumed I know a lot more than I do. I presume one must first go to Layers > Add New Layer which I have apparently done. To "Make a black & white gradient-filled shape on [this] separate layer" must I then use the Rectangle Select or Lasso Select tool? Below is the result although the lasso seems to have disappeared. As for the further instructions, I'm afraid that without step-by-step novice instructions I haven't a clue how to proceed. And perhaps this is too much to hope for.

Apologies if this question has already been submitted. In a rectangle, or, better, a polygon at the top of a dawn photo for the portal of a website I want to implement a lightening gradient to set off the site's logo. Any fool-proof instructions for how to accomplish this will be deeply appreciated. Thank you.