This is what I used for featuring an image in the feed display ("today" tab), it matches the first image in the feed content body

image-in-feed.rb

recordset.each do |entry|
  # Some other stuff
  # ...
  
  image_field = 'content' # 'description' in rss
  
  unless entry[image_field].nil?
    img = entry[image_field].match(/<img.+?src=[\"'](.+?)[\"'].*?>/)
    unless img.nil?
      item[:image] = raw.merge(img[1]).to_s
    end
  end

end