iTunes Custom RSS Tags - Image
The iTunes image tag is one of the few tags added by Apple with any substantial value over the base RSS 2.0 specification. The RSS image tag is limited to maximum dimensions of 144x400, which seems fairly non-standard and certainly doesn't match the square dimensions of the album art displayed in the iTunes Music Store. Maximum dimensions for the iTunes Image tag are 300x300. It's recommended to use the maximum. This is one tag you should definitely include to make sure your podcast is differentiated from the default image in iTunes. Supported file formats are JPG and PNG. Apple seems to have two different methods for linking their image tag. Both appear to be correct.
The first method for including an image is the standard:
A second method shown in Apple's example is using the <itunes:link> tag to imbed the image:
<itunes:link rel="image" type="video/jpeg" href="http://www.YourPodcastURL.com/YourImage.jpg"></itunes:link>