Tested this on an Windows/IE PC. A very similar method should work on OS X.
- Open the TV show in the iTunes store
- Right-click on the artwork and 'Copy Link'
- Paste the link into IE, and wait for a page like this to load: http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewTVSeason?id=192598886&s=143444
- Right-click on the artwork there, and choose properties
- Copy the URL (Something like http://a1.phobos.apple.com/us/r1000/011/Features/c8/da/06/dj.wwdotahp.100x100-75.jpg )
- Paste into the address bar and change 100x100 to 600x600
- Enjoy!