How do I add an image to an item in RSS 2.0?

This is possible in RRS2,

see http://cyber.law.harvard.edu/rss/rss.html#ltenclosuregtSubelementOfLtitemgt

So you have to use the enclosure tag, to add media

One of solutions is to use CDATA in description

<![CDATA[
  Image inside RSS
  <img src="http://example.com/img/smiley.gif" alt="Smiley face">         
]> 

Note, that you may have a problem with hotlink prevented site.

The enclosure element can be used to transmit pictures. The RSS 2.0 spec is quite clear about that, saying that the type is a MIME type. It does not say it is restricted to audio or video.

Here's an example: a set of photo feeds from Agence France Presse