Resizing banner image for mobile

Hello, 

I would like to resize this banner image for mobile viewing or be able to show a different photo for mobile viewing. Does anyone know of the correct coding for this. We're using 7.0. 

Thank you!

Kayla

What is site url? We can check easier

Sorry I thought I included the link:

https://www.bsw-in.com/meet-your-bloomington-pain-rehab-team

Try this code to Custom CSS box

@media screen and (max-width:767px) {
body#collection-5b76d777c2241b2cfa25ea1a .Parallax-item:first-child img {
    width: 100% !important;
    left: 0 !important;
    height: auto !important;
}
body#collection-5b76d777c2241b2cfa25ea1a section.Intro.Intro--has-image .spacer-block {
    display: none;
}}