remove the favourite and re-add it.
Seemed like a logical solution to me, so I tried it but got an unexpected result.
When I select a podcast from the favorites list that has a missing icon, the option is to add it rather than remove it. If I add it, it adds a second copy of the same podcast with the icon, but the original missing icon entry also remains. If I select the remove option from either copy, the newly added duplicate with the icon is removed, but the original with the missing icon remains and the option reverts back to add rather than remove.
There doesn't appear to be any way to remove the podcasts with the missing icons short of doing a factory restore of the ATV. I'd say this is definitely a bug in the ATV software.