You can upload artwork at the series or episode level in ART19. In order to upload series artwork, navigate to the "Image" card on your series page. Click the plus sign in the top right corner, or click "Add Image" in the middle of the card. Your series image must have a 1:1 aspect ratio and a minimum resolution of 1400x1400 pixels. To be eligible for featured placement in Apple Podcasts, artwork must be 3000x3000 pixels.
You can also add an image at the episode level. If you choose not to add an episode specific image, your series level image will default to the episode level.