How can I assign an image to a podcast in iTunes? In the podcasts list in iTunes, some podcasts don't show an image.  Is there a way to assign one?

Online help in iTunes suggests doing a Get Info and clicking the Artwork tab, but that tab is always dimmed, even for podcasts that do show an image.

Attempting to Google this mostly shows things about how the creator of a podcast can provide an image.  But I want to assign an image when the creator didn't.
It seems to me that unfortunately there is no way of assigning custom artwork to subscribed podcast trough iTunes.