You have to encode the image as base64 data, then use this xml tag (i have it at the very end, just before the final closing tag).
… base 64 data here …
You can encode the image at the URL below. Use a PNG ideally, and check the compress option.
http://www.askapache.com/online-tools/base64-image-converter/
Copy the “raw” output, then use only the data part (everything following “data:image/png;base64,”).
Make sure your image is the same size as the other thumbnails, otherwise the base64 data will be very long.
Good luck 