Apple has just informed us here at the IAIB that it will start requiring all iTunes (podcast) cover art to be a minimum of 1400x1400 pixels and a maximum of 2,048 x 2,048 pixels. It was a little over two years ago when iTunes suggested podcasts to add a 1400x1400 cover art in order to be considered for promotion in the iTunes Store and Podcasting application for iOS

Along with this suggested change Apple has also suggested that we no longer save our image art in .png format.

Via Apple :

Why do I need cover art?

"High quality cover art attracts new subscribers to your podcast. We recommend including a title, brand, or source name as part of your cover art."

"Cover art must be a JPEG or PNG file in the RGB color space with a minimum size of 1400 x 1400 pixels. For best results, and to be considered for promotion in the iTunes Store or the Podcasts app for iOS, cover art must be at least 1400 x 1400 pixels."