“Podcast cover art must be at least 600x600 pixel jpg or PNG, in RGB color space, and hosted on a server that allows HTTP head requests”
If you are getting this error, this means you don't have an image attached to your blog. You need to have one for iTunes to accept your submission.
To add an image to your blog:
- Go to Site Administration
- Go to Content Controls -> Blog Administration
- Click on the name of the blog you're submitting
- Once the blog loads, hit the Edit Blog icon in the top left hand corner of the Admin Bar
- Go to the Display tab and upload an image where it requests one.
- Save your changes.
- Test your feed again.
If you are getting other errors, check Apple's pages here: http://www.apple.com/itunes/podcasts/