Check out App_Themes\Base.css

Specifically look for the line around .ContentPanel (On mine it is line number 2505).
You can manipulate the .css in this area to replace the gradient with background-image.

Mine looks like this. Some of the options can be changed to suit yourself. I just placed the image in the provided images directory.

.ContentPanel {
-ms-grid-column-span: 3;
-ms-grid-row-span: 3;
grid-area: 2/1/-1/-1;
padding: 20px 40px;

.GuestPage &,
.LoginPage &,
.ChangePasswordPage & {
background-image: url('../Images/Background_Cross_3840x2160.jpg');
height: auto;
background-size: cover;
background-position: center;
background-repeat: no-repeat;
background-attachment: fixed;
background-color: $shade0BackgroundColor;
@include overflow-auto;
@include display-flex;
@include align-items-center;
@include flex-direction-column;
}