I can't answer why this has happened: the image is correctly referenced in the feed. However it's a GIF; strictly speaking Apple's specification says it should be a JPEG or a PNG - however if it's been working up to now that doesn't necessarily explain things.
Anyway, I should change the image for a JPEG - ideally it should be 600 x 600 - and hope that that does it: expect it to take some days before the image appears.
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...
<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>
<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>
The part of the feed you've posted seems to be fine: the image is correctly referenced and loads fine, so there's no problem with Feedburner.
Your podcast appears to be at http://itunes.apple.com/gb/podcast/chec-music-with-jake-chec/id372456706
and your feed is presumably the one at http://feeds.feedburner.com/jakechec/checmusic
the image appears on the Store page; so I can't offer any comment as to what the problem you describe is caused by.
I can confirm that the image is not showing in iTunes though it's correctly references and can be viewed from the URL in a browser. I can't offer any explanation for this other than it's being a bug in the Store. There isn't really anything you can do about this except email podcasts 'at' apple.com and ask about it, though I don't know what chance you've got of getting a helpful answer.