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This is its default thumbnail. Its screenshot is as given below:
You can locate the URL of this default image embedded in the HTML of the template. To get its URL, right click on the image>Copy Image Address>Go to Dashboard>Template>Edit HTML>Find (Ctrl+F) that URL.
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One its URL is being located, you can replace it with your own image. And the image you want to add should be also in the similar format (URL form) like their default thumbnail URL.
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