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'/>
          </div>
      </b:if>
        <data:post.snippet/>
      <b:else/>
        <data:post.body/>
      </b:if>
  <b:else/>
  <data:post.body/>
  </b:if>

```

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?