In your post admin under the description section, there’s a “Your Theme - Video information” metabox. You will find here a Thumbnails section with a Add Thumbnails button:

Then in the media popup you can upload your images and select the ones you want to use for the thumbnails rotation:

Click on “Choose images” and you will see the images selected in the Thumbnails section:

It’s done! You have manually set thumbnails rotation for your post. Don’t forget to update your post to save the changes.

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