How about we stay on topic and help the individual with his requested support for Windows Media Center as opposed to adding other programs?
Turn both updates and optimization on in WMC and give the system time to run the daily updates (broadband speed will greatly help) and to complete indexing - if the movies are legit and not downloaded versions you should notice that WMC will add the artwork assuming you have added your collection to the video library. Also verify that write permissions are default on the drive/folders. If you have blocked permissions or write access to system, for example, the folders cannot be updated with artwork.
That being said, even though Windows 7 WMC supports more movie formats, not all are...what are the file types/extensions?