I have a similar problem. I uploiad a 600 by 600 jpp image and all works fine for a few days. Then, its just vanishes, ***!! Here is part of my xml referance...
href="http://pubsubhubbub.appspot.com/" />
<media:thumbnail url="http://www.jakechec.com/radio/jclogo2012red.jpg" />
<media:keywords>chris,lake,thomas,gold,dj,chus,darude,dj,dan,d,ramirez,daniel,po rtman,an21,jelo,jake,chec,chec,music,house,vocals,commercial,house,alex,gaudino, pan,pot,michael,woods,deadmau5,dunity,john,dalhback,live,club,recordings,house,m usic,free,downloads</media:keywords>
<media:category scheme="http://www.itunes.com/dtds/podcast-1.0.dtd">Music</media:category>
<itunes:owner>
<itunes:email>info@jakechec.com</itunes:email>
<itunes:name>Jake Chec</itunes:name>
</itunes:owner>
<itunes:author>Jake Chec</itunes:author>
<itunes:explicit>no</itunes:explicit>
<itunes:image href="http://www.jakechec.com/radio/jclogo2012red.jpg" />
<itunes:keywords>chris,lake,thomas,gold,dj,chus,darude,dj,dan,d,ramirez,daniel,p ortman,an21,jelo,jake,chec,chec,music,house,vocals,commercial,house,alex,gaudino ,pan,pot,michael,woods,deadmau5,dunity,john,dalhback,live,club,recordings,house, music,free,downloads</itunes:keywords>
<itunes:subtitle>Jake Chec presents the worlds finest djs and producers.</itunes:subtitle>