For podcast and episode artwork, try to aim for the following when uploading to ZenCast:
3000 x 3000 pixels resolution (at minimum 1400 x 1400)
JPEG or PNG file format
File size less than 2 MB
If your show's artwork file doesn't meet these guidelines, we will try to optimize it for you. However, we are unable to guarantee that it will look as good as it can, and there may be some distortion or visual artifacts as a result.
So, when possible, it’s better to resize it yourself and make sure that you’re happy with the results before uploading.