Help with my Podcast feed

Hi there. Ive had a podcast feed with feedburner for a few years now. At the start it was fine but then it stopped working. I got it fixed but my podcast stopped showing up in the iTunes store. thats been about two years now. Although I have been adding new episodes every week and there have been going into iTunes. But last week they stopped going into iTunes all together and all my episodes have dissappeared from iTunes. I can still add them and update my feedburner so they appear on my website. but I can not get them back into iTunes and cant work out why. The message I got when I tryed to re-submit is "Podcast Cover art must be at least 600 x 600 pixel JPG or PNG, in RGB color space and hosted on a server that allows HTTP head requests" my website is www.ruddsrecords.co.uk and if some one could have a look at my code that would be great. It was a friend of mine who set the feed all up for me and im not as technically gifted.

Thanks

Stevie McCoy