.fbx-modal div.fbx-loader div { display: none; }
.fbx-modal div.fbx-loader {
  background: url('https://my-site.com/custom-loader.gif') no-repeat;
}

Walkthrough

Replacing the FooBox animated loading icon with your own animated gif is pretty straight forward. Simply use the CSS snippet above and paste it into your “Custom CSS” field in the JS&CSS tab of your FooBox settings.