[1] 仕様書:
-[CITE@en[RSS 2.0 Specification (version 2.0.10)]]
--<http://www.rssboard.org/rss-specification#optionalChannelElements>
--<http://www.rssboard.org/rss-specification#ltimagegtSubelementOfLtchannelgt>

[2] [[子要素]]:
- [CODE(XMLe)@en[[[description]]]]
- [CODE(XMLe)@en[[[height]]]]
- [CODE(XMLe)@en[[[link]]]] (必須)
- [CODE(XMLe)@en[[[title]]]] (必須)
- [CODE(XMLe)@en[[[url]]]] (必須)
- [CODE(XMLe)@en[[[width]]]]