<image></image> is an optional sub-element of <channel> in RSS 2.0 and I believe you can use that. It has 3 required sub-elements:
<url> (The URL of the image)
<title> (describes the image)
<link> (link of the SITE)
and 3 optional sub-elements:
<width> (default is 88-max 144)
<height> (default is 31 - max 400)
<description> (text for the title attribute of the link to the image)
So, you could structure your feed like this:
<image>
<url>http://www.yourwebsite.com/yourimage.jpg</url>
<title>Title of image</title>
<link>http://www.yourwebsite.com</link>
<width>50</width>
<height>150</height>
<description>This is my cool image for my feed</description>
</image>