So you finished the latest update to your Neocities website, uploaded all your new pages, even saving the best one till last so it shows as the main thumb on your "site was updated" thing on your profile... aaaaaaaaaaaaaaaand....
And then...
Well, I have a fix! Next time this happens. Make a file somewhere on your computer called redir.html. Copy the following code, and paste it in there.
<!doctype html>
<html>
<head>
<script>
window.location.href = "https://yoursite.neocities.org/path/to/preferred-page.html";
</script>
</head>
</html>
Next time the above happens, change the window.location.href line so that the URL is the URL of the page you want to be the main thumb.
Then upload the file.
It it doesn't work, give it a couple of minutes, rename the file on your machine to redir2.html, and try again. It may take a few attempts, as the Neocities thumbnail choice seems random these days.
Once a couple of days has gone by and your update has been uh, "absorbed into the past", You may want to delete the redir.htmls so you can do this trick again without ending up with a whole bunch of redir.htmls.
Additionally, something tells me now I've posted this the very next thing Neocities is going to do is have its thumbnail generator ignore redirects, so I dunno how long this trick will work for.
Hope that helps someone out!