All in One SEO Pack gives you control over the SEO Meta Data on all images you add to your site just like it does with Posts and Pages. These settings are found at the bottom of the Edit Media screen for the specific image you are editing.
When you are adding an image to a Page or Post it is important to add it as an Attachment Page (see screenshot below). The Attachment Page is then treated as content within the site, rather than simply an image file. This Attachment Page will have the SEO Meta Data you added in the Edit Media screen.
If you chose not to edit any of the SEO Meta Data, then title of the image will be used as the Meta Title and the caption (if there is one) will be used as the Meta Description. If there is no caption and nothing set in the Description field of the All in One SEO box then the Meta Description will simply be blank.