Your podcast cover art is the square image that listeners see on their listening devices and on most podcast players.
Each podcast can have a couple of types of artwork:
Podcast show artwork - the cover art for the podcast itself, the main cover; and
Podcast episode artwork - cover art specific to each episode, shown in more and more podcast players on an episode-by-episode basis
Luckily, both pieces of artwork follow the same specification.
Here's the official specification from Apple:
Create artwork for your podcast that still works well when scaled down to thumbnail size on mobile devices. Before you create your podcast artwork, review the Top Podcasts chart for examples of compelling artwork.
To be eligible for featured placement in Apple Podcasts, artwork must be a 3000 x 3000 pixel JPEG or PNG file with 72 dpi in the RGB color space. You can also use specific images for individual episodes.
Note: To make sure artwork elements aren’t obscured by play progress and played indicators, avoid putting artwork elements in the bottom fifth of your image.
To summarize, your artwork must be:
Max. 3000 x 3000 pixels in size (min. 1400 x 1400 pixels)
Set to "RGB" setting
Ideally under 512kb in filesize
From experience, don't put anything naughty on the artwork, either. They won't like that. That means:
No swearing or explicit language
No explicit images
No offensive or discriminatory imagery