In your theme’s head file you can add this:
<script type="text/x-handlebars" data-template-name="/connectors/below-site-header/custom-banner">
// Your custom HTML here
</script>
This utilizes a plugin outlet to put the content directly below the header and outside our content container (more details here: Developer’s guide to Discourse Themes)
Based on the variable name… is this an SVG image? If it is you may need to make sure the SVG file has a viewbox and/or dimensions in its markup… I’ve occasionally run into this problem.
Otherwise, I’d expect your image to appear… is this happening on a live site, or on a local development install? I’ve occasionally had issues with uploaded images on dev installs, usually clearing the cache or restarting the server fixes it.