ITunes Movies Cover Art - please help!

I ripped about 100 movies in mp4 format. When I import them in Itunes, it grays out the Cover art Tab, and i am unable to click the tab to change the cover art.

For movies that i download from itunes store, it allows me to do so. Please dont tell me that Itunes only allows cover art for movies that are downloaded from the itunes store.

Can someone please advice ? Thanks much