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()

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.first.getUrl() }}" />
{% endfor %}

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