There is the simple way to customize your blog with automatic thumbnail and read more on the content. The steps are as below:
1. Login to blogger.com dashboard.
2. Go to Edit HTML and Expand Widget Templates
3. Find below code
<data:post.body/>
4. Replace <data:post.body/> with below code.
<p><script type='text/javascript'> summary_noimg = 430; summary_img = 340; img_thumb_height = 100; img_thumb_width = 120; </script> <script src='http://blogergadgets.googlecode.com/files/excerpt.js' type='text/javascript'/> <b:if cond='data:blog.pageType != "item"'> <div expr:id='"summary" + data:post.id'><data:post.body/></div> <script type='text/javascript'>createSummaryAndThumb("summary<data:post.id/>");</script> <span class='rmlink' style='float:right'><a expr:href='data:post.url'>Read more</a></span> </b:if> <b:if cond='data:blog.pageType == "item"'><data:post.body/></b:if></p>
5. Click Save.