Text of Image format posting. Link to an external Hubble image follows, including title, alternate text, and caption:
Here’s some more text that immediately follows the image. Note that the optional caption appears in the surrounding background area, below the image. I elected to have the image be a clickable link directly to the source image. (By default, clicking on the image itself does nothing. I could easily have specified any destination URL for a click on the image, with the option to open in a separate window.) This format of post renders all of the elements that show up in standard format, with the addition of “IMAGE” above the title in the heading that appears in the main blog page and applicable archive pages. Another difference from other formats is that the additional information below this type of entry includes the posting date, and this is shown against a distinctive, rectangular background.
What http://codex.wordpress.org/Post_Formats says about this format:
image - A single image. The first <img /> tag in the post could be considered the image. Alternatively, if the post consists only of a URL, that will be the image URL and the title of the post (post_title) will be the title attribute for the image.