When you say you have "cut and pasted" a new image, you are talking about from MC's cut/paste menu options, right?
Since you say you say the correct cover art shows when you edit the "first file", try this.
-Right click on this "correct" image in MC and choose cover art/copy to clipboard from the menu
-Right click on the image in MC, yes... again, and choose cover art/paste from clipboard from the menu
I know this seems redundant and silly, but for audio cover art at least, not doing the second step will leave all the remaining tracks with whatever image they had previously that you wanted to replace. I presume this is to allow different cover art for different tracks of an album. Not sure if that is the intent, but it is the result. Granted, MC handles movie cover art differently than audio, so this may not work. That said, it only takes a few seconds to try, so why not give it a shot.
A couple other things to try:
- copy the image file to the clipboard via Windows Explorer (instead of MC copy from the menu). Then use MC's paste from clipboard. At least with audio, when I use the copy command from outside of MC and then paste it, it applies the image to ALL tracks, not just the first file.
- and finally, perhaps the most obvious option of all. Have you tried simply right clicking on the image and choosing Get Movie & TV Info and then choosing a new image from that list to see if that sticks? I imagine you have, but figured I should cover all the bases.