Banner Image Sizing Issues

When I add an image to my banner, it does not cover the full length and width of the banner area as it should. The result is that the black border can be see on certain screens. For example, I do not see black border when bringing up the site on my iPhone, but there is a significant amount of black border seen on my (and others) laptops.

I’ve tried using an image widget and text widget, changing pixel size of the picture, and adjusting the boxed width of the site. Nothing fixes it.

Please let me know what I need to do to adjust my banner so that the picture completely covers the area on all device screens.