Here's guidance from Apple regarding iTunes podcast imagery:
iTunes recommends using a 1400x1400 pixel .jpg image.
"Create a graphic for your podcast that is easy to recognize when scaled down to 50x50 pixels. Good art communicates the value of the podcast with a simple picture and a few words. Before you create your podcast art, go to the Podcast page in the iTunes Store, click on Top Podcasts, and note which art works best and why. We recommend a 1400 x 1400 pixel JPG for cover art."