The posts where you see a grey background and the tags, date, etc. below the post are using the image post format.
You can see the same thing on the demo site if you scroll down to see some of the image-only posts:
Twenty Eleven detects when a post consists only of an image, automatically assigns the image post format to them, and displays them a little differently. You can learn more about post formats.
There is a workaround you could use to "trick" WordPress into not automatically assigning image-only posts the image post format.
Below your photo in the Visual Editor, flip to Text view, and then add this code:
<span style="color: #fff;">.</span>
What this does is add a white period to the post, effectively tricking the theme into thinking there is content other than just an image, and forces the post to have the standard post format.
Let me know how it goes.