Add these styles to your design CSS overrides to add a background image to your Helpy

helpybgimage.css

.flat-main-panel, .main-option {
  background-color: transparent;
  color: #fff;
}
.flat-main-panel {
    background-image: url('http://your.helpy.io/uploads/your/image/file/3/notebook-1280538_1920.jpg');
    background-repeat: no-repeat;
    background-position: right top;
    background-attachment: fixed;
    background-size: 100%;
}
.home-index #home-search, .home-index #how-help, ul.breadcrumb {
  background-color: transparent;
}