While I can not unlock the series (as this has nothing to do with images at all) I could mark the images as replaceable, however I suspect you'd be disappointed. Maximum thumbnail size for any episode is still restricted to 400 pixels wide by the appropriate and proportionate height. Perhaps on the new site we'll allow larger thumbnails, but they won't be uploadable on the current site. If you'd still like to upload replacements despite the limitations please let us know. I'm also moving your post to the correct section of the forum.
Yes, I would like the ability to upload replacments.
I did a test, with S01E01, and here is the TVDB current image and the new one at 400 pixel wide.'
(I attempted to match the frame exactly as i could)