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I recently started with blogger. I was tweaking the homepage a bit to display thumbnail and post excerpts with read more link.

A lot of websites say to use this kind of code:

  <b:if cond='data:blog.pageType != &quot;index&quot;'>
      <b:if cond='data:post.snippet'>
      <b:if cond='data:post.thumbnailUrl'>
          <div class='Image thumb'>
            <img expr:src='data:post.thumbnailUrl'/>

But can we tell blogger what's in data:post.thumbnailUrl? For some people, blogger will just pick an image magically from their post.

How can I decide which thumbnail will be used?

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up vote 2 down vote accepted

You can't control this, Blogger decides for itself which image to use.

I did some reasearch a while back (documented here), and concluded that it's most probably the first pictures in the post which is both

  1. over a certain size, and
  2. hosted in your own Picasa-web-albums / Google + Photos albums.

From memory, I think that testing showed that the size was 17k. But I'm not 100% sure if that rule is universal - it may depend on the picture dimensions, too, since I cannot see that a very tall or wide thumbnail would be overly useful.

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You are right, I did more digging after that and I could not understand why my first and unique image in my post was not taken as thumbnail. It turns out that it was because I inserted this image with a link to my Amazon S3. I don't understand why there is no official doc about this. Thanks for your answer. – Amida Mar 8 '13 at 11:52

Blogger doesn't pick images by magic. Blogger will automatically choose the first image as a thumbnail. So if you need to keep your own image as thumbnail, please make sure that the desired image is kept first while you write the post.

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Yes but the problem, as I mentioned in a previous comment, is that it did not work because the image was not hosted in Picasa / Google img related services. Something that is (was?) totally not indicated anywhere. – Amida Oct 1 '13 at 14:40

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