I just looked at your site and saw, that your header slider has a max-width of 50%. Did you wrote this css? I think it should be removed or overwritten with
max-width: 100% !important;
But to come to your question, maybe try to set up the image with a width of 1920px and height of 1080px. If the image is just that small as it is now, you can create a image with the size of 1920*1080px with a white background and paste in your image in a programm like Photoshop or GIMP.
I also noticed that your header image on the other sites with the tennis balls is pretty the same color as the links are, they are hard to read.