How can I properly import an image without an alpha channel?

Apple requires that the Top Shelf Image asset for Apple TV not have an alpha channel. My source image has dimensions of 1920x720 and it's in a 24-bit JPG format.

I've set the "Alpha Source" import setting to "None" which I assumed imported the image without an alpha channel. But when the "Max Size" for the image is set to 2048, the image that Unity generates during the build gets generated with an alpha channel and makes Apple reject my app.

If I set the "Max Size" to 1024, then it generates it without an alpha channel correctly. I'm concerned that doing this could reduce the quality of my image. Is this a Unity bug or am I doing something wrong? Is this an issue with Top Shelf Images or with images in general?

I'm using Unity 5.5.0f3.