Consider using a citation manager to keep track of the images you use. The library supports Refworks.
Citing Images in Chicago/Turabian Style
In general, your citation should include information about both the image and the source of the image. When elements of the description are missing, provide enough information so that the image can be traced to its source. You may leave out the author name if none is provided. If no title for the image is provided, create a descriptive title framed in quotes.
If the image is your own, you could cite it as unpublished resource. On the other hand, you could also post it online (with the appropriate Creative Commons license, of course), and cite from the resulting page.
Citing Images in Turabian
From a print source:
Artist’s name (first then last), title of the artwork (italicize titles of paintings and sculptures but set all other titles in quotation marks), creation date, repository name (including city and state).
If the artwork appears in a published source (book, magazine, etc.), give the publication information in place of the repository or location. Include the page number.
Georgia O’Keeffe, The Cliff Chimneys,1938, in Barbara Buhler Lyens, Georgia O’Keeffe and New Mexico: A Sense of Place (Princeton: Princeton University Press, 2004), 25.
From a digital (Internet) source:
Artist’s name (first then last), the title of the artwork (italicize titles of paintings and sculptures but set all other titles in quotation marks), creation date, repository name (including city and state). Title of the online site, format type, URL, access date (in parentheses).
Mark Rothko, Orange and Red on Red, 1957, The Phillips Collection, Washington DC. Artstor, database, JPG, www.artstor.org. (accessed September 20, 2013).
Citation & style guides
These works provide detailed information on citation styles. See the examples below.
Electronic version of one of the most used style guides for writing in American English. Provides rules and suggestions for bibliographies and other forms of documentation such as parenthetical references, footnotes, and endnotes. Includes instructions on how to cite electronic and online sources. Also contains a chapter on grammar and usage (punctuation, spelling, etc.).