.... a few more tips.

Before adding your sky photo to the 2D plane, set the plane to a nice bright material (one of the simple and fast, like red, green, or blue work nicely.)

Make sure the 2D plane is perpendicular to the camera.  As you move it back to get it behind the other objects in your scene, that bright color is easier to see.  You want the 2D plane to cover all the open sky area, but be as small as possible to accomplish that. 

When you get the plane positioned, you can not change from the bright texture to the photo.

Even with the atmosphere off, the sun position is very important -- not just for your scene shadows, but for how bright your sky photo looks.

Some objects may cast shadows to your sky photo.  Select those objects and turn off the "cast shadows" if you want to avoid that.

This same procedure can be used to make "camera filters" or to simulate something like rain.  Here you would bring the plane up to the front and use transparency.

I like to make scenes without post processing, so I made my signature/copyright into a 2d plane object, which I can add to the render.