The Aerva RSS widget is compatible with RSS 2.0.  The most important elements of the structure are as follows:


<rss xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:media="http://search.yahoo.com/mrss/" version="2.0">
   <channel>
      <item>
         <title>...</title>
         <description>...</description>
         <media:content url="..." />
      </item>
   </channel>
</rss>


Here's an example of a feed with two items in it that is fully compatible with the Aerva RSS widget.  Feel free to use this as a reference:


<rss xmlns:media="http://search.yahoo.com/mrss/" xmlns:dc="http://purl.org/dc/elements/1.1/" version="2.0">
    <channel>
        <title>My Feed Title</title>
        <link>http://example.com/myfeed.xml</link>
        <description>My Feed Description</description>
        <language>en-us</language>
        <pubDate>Tue, 02 Aug 2016 15:50:39 -0400</pubDate>
        <lastBuildDate>Tue, 02 Aug 2016 15:50:39 -0400</lastBuildDate>
        <item>
            <title>Item 1 Title</title>
            <link>http://example.com/myfeed.xml</link>
            <description>
                Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.
            </description>
            <pubDate>Fri, 29 Jul 2016 14:30:00 -0400</pubDate>
        </item>
        <item>
            <title>Item 2 Title w/ Image</title>
            <link>http://example.com/myfeed.xml</link>
            <description>
                Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
            </description>
            <pubDate>Fri, 29 Jul 2016 14:30:00 -0400</pubDate>
            <media:content url="https://upload.wikimedia.org/wikipedia/commons/5/55/Aerva_lanata_in_Bhongir_AP_W_I2_IMG_3064.jpg" type="image/jpeg"/>
        </item>
    </channel>
</rss>