Media Center Thumbnail Image Size

Although Windows 7 media center does a very good job of creating thumbnails for all the videos, sometimes they’re just not good enough.
Luckly there is a simple answer. Simply put a JPG file in the same location as the movie file, with the same name (but different extension of course) and media center will use that image as a thumbnail.
However, if you’re looking for good thumbnails you can’t use just any image. It has to be the right dimensions. The actual image size appears to be 320×420, but if you use an image that is 1024×768 it gets scaled down nicely as well.
If you don’t have ready access to an image for your movie, heres a tip for using google to find the right image size.
If the search field type groundhog day imagesize:1024x768
That will return all images relating to “groundhog day” of the size 1024×768

Source : How to add thumbnail images to Media Center video library files « Media Serve.