You'll need to upload an image for your podcast logo. The specs for this image are:
1400x1400 px Minimum (Square) jpg
3000x3000 px Maximum (Square) jpg
iTunes recommends artwork be in the JPEG or PNG file formats and in the RGB color space with a minimum measurement of 1400 x 1400 pixels and a maximum measurement of 3000 x 3000 pixels at 72 DPI.
The size should be 512kb (while iTunes does not recommend an image size, this meets the requirements for NPR One and Spotify).