Yeah, I think you are close, you've just go your variables a bit mixed up. You set `entries`, but then reference `entry` and I think `getUrl` should be called as a function `getUrl()` as it's not a property (it might work that way, I've never tried tbh)

    {% set entry = craft.entries.slug('homepage') %}
    
    {% for image in entry.podcastItunesImage %}
     <itunes:image href="{{ image.getUrl() }}" />
    {% endfor %}

**Edit**  
Oh I don't think you need `{{ siteUrl }}` when you call `getUrl()`