Slider image sizing issues

I have just installed the Virtue theme (2.6.7) and am working through setup. I am working on the slider image (first one) and I cannot get the sizing to work right. I have used various sizes of of the same image and none seem to fit right to where it shows the whole image. I have tried 1920×1200 down to 900×400. What is the best image size to use and what might I be doing wrong?

https://www.familyworshipradio.com

Thanks,

Lee