[Theme Horse Support Team]

Hi Laura,

First what you have to do disable the slider and remove the image from the slider and set that image as background image.
To set the background image go to Appearance > Background select the image and set the settings as shown in this screenshot and again go to the Appearance -> Customize/Theme Options -> Design Options Tab -> Custom CSS paste the following CSS code and Click on save all changes button:

#site-title a img {
    max-width: 300px;
}
body {
    background-size: cover;
}
#colophon {
    bottom: 0;
    position: fixed;
    width: 100%;
    z-index: 1000;
}

Thank you!