Because I use the excerpt box as I described in another thread . The theme has an activated excerpt box. You can over ride the excerpt provided by the theme by using it.
On my blog I chose the "concise" format. I also close to insert "the more tag" into the latest post. The “concise” format displays only the title and the first 20 words from each post so that your readers may view your posts at a glance.” http://theme.wordpress.com/themes/chateau/
I choose “concise” and I copied the code from the HTML for the thumbnail image and the text teaser sentences of the length that I wanted into the excerpt box below the editor box. In other words, I overrode the excerpt that's coded into the theme by using the excerpt box. . It was easy to do. http://en.support.wordpress.com/splitting-content/excerpts/
If you are doing this when you create a post then after your create the post and before you publish it go to the bottom of the post, upload a thumbnail and copy and paste the code from the HTML editor into the excerpt box, then delete the thumbnail image from the post. Then copy and paste the teaser text into the excerpt box and publish the post.
Or if you prefer open another tab and click new post to create a draft post you will be deleting, upload the thumbnail into it and copy the code from the HTML editor. Go to the first tab you still have open and paste the thumbnail code onto it. Go back to the draft and delete it. Copy and paste the teaser text into the excerpt box and publish the post.