Excerpt from: Podcasting at Work: 25 Questions to Ask Before Starting

Your podcast artwork is the visual that will accompany your show in every podcast app. Keep in mind that potential listeners may encounter many different pieces of artwork when they search in podcast directories, so make sure yours is quickly understood, and visually enticing.

Podcast artwork should be square and measure 3000x3000 pixels. This is quite a large image, but don’t be fooled into thinking you’ve got a lot of space to work with. Podcast artwork is often displayed in apps around the size of a quarter or even smaller. So use limited text and make it big and bold! Take a look in your favourite podcast app for inspiration and consider how to make yours stand out from the crowd. Apple has further artwork guidelines and recommendations here.