Make sure it works in Apple Podcasts.
While Spotify, Google Podcasts, and other podcast directories are a great way to reach new listeners, Apple Podcasts (formerly iTunes) is the largest and most popular directory. It is critical that your podcast is listed in Apple Podcasts and that your artwork meets their specifications.
Excerpts from iTunes Feed and Image Requirements:
[Cover art must be] a minimum size of 1400 x 1400 pixels and a maximum size of 3000 x 3000 pixels, 72 dpi, in JPEG or PNG format with appropriate file extensions (.jpg, .png), and in the RGB colorspace. To optimize images for mobile devices, Apple recommends compressing your image files.