An image from an electronic source can be cited according to the Chicago Citation Style 15th Edition in three different ways.

The format for the full note is:

Author First Name/Initial Surname, Image Title, Year, URL.

The format for a concise note is:

Author Surname, Image Title.

And the format for the bibliography is:

Author Surname, First Name or Initial. Image Title. Year. URL.

As an example consider this image taken from the website Flickr.com.

The citation in full note in this case would be:

Buck Lewis, NWO, 2012, http://www.flickr.com/photos/thephotoholic/7009047673/?f=hp

The concise note for the same image is:

Lewis, NWO.

And the bibliography is:

Lewis, Buck. NWO, 2012, http://www.flickr.com/photos/thephotoholic/7009047673/?f=hp