how to change artwork on my itunes podcast

You can manually change the artwork for it by right-clicking the podcast and click get info. From there you can edit the artwork of said podcast and add whatever artwork you want. Hope this helps you.

I assume you are talking about a podcast you have created yourself (in which case it's better to ask in the Producing Podcasts forum).

When you change the image whose URL appears in the 'itunes:image' tag in the top section of the feed (before the first 'item' tag) you must make sure that you give it a new filename, since the Store caches the image and won't know that it's changed if you keep the name the same.

Once the URL has been changed in the feed and uploaded it will take some days for the Store to get round to changing it. You should check that the URL has appeared in the feed and that the image itself is compliant with requirements - minimum 1400 x 1400px, JPG or PNG, RGB, and less than 500 kB. Images which do not meet this specification will be ignored.