If the default background image works on this landing page as expected, it confirms that the issue lies with the custom coding that was added to the template. Marketo Support is not set up to assist with troubleshooting custom code. The best point of contact will be your web developer who should be able to resolve the issue for you. Please be aware that the landing page editor may not load all external resources in the editor or previewer, which can cause them to look different than the published version.
If the background image doesn’t render on this newly downloaded template as well, contact support and we will assist in troubleshooting your issue.