haven't read through this entire thread so this may have already been covered, and it looks like the getvideoartwork website should suffice for most people, but…
1) go to the show page in itunes and select "copy link"
2) paste in to safari and you'll get to a page like this
http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewTVSeason?id=284583188&s=143441
3) right click on the image and select "open image in new tab" and you get a page like
http://a1.phobos.apple.com/us/r30/Features/dd/77/83/dj.vahzbvgm.100x100-75.jpg
4) simply replace the 100x100 with 600x600 and you get the full size artwork
probably too little too late or already covered but it might help.