Thank you to everyone who let us know about iTunes' updated requirements. Hipcast already supports Byte Range Requests and you will not need to change anything on your setup to keep your podcast on iTunes. We will support the new image sizes, but they are not required by iTunes for your podcast to keep working, and you don't need to upload a new image. iTunes only requires the larger image if you are featuring your podcast in the iTunes store.
We are working on several updates including support for the larger image sizes in iTunes, coming soon.