New blogger XHTML, post first image (thumbnail)

When i edit my blog template, i found a code that i never seen before. That code is post  first image. The code it's like an old blogspot code data:post.thumbnail, but the old code resolution is 72px, but this new code's resolution is like the src image resolution. So this is very good for a template post thumbnail.

The code

I will no give a tutorial, i just tell you the code
<data:post.firstImageUrl/>
That code will only work in the 'blog post' widget.