If you reference an image in your research, you should cite the image properly using the same referencing style you are using for the rest of your assignment.
Best practice in this case is to include the whole reference in a caption under the image.
Fig. 1. Man exercising from: Green, Annie. "Yoga: Stretching Out." Sports Digest, 8 May 2006, p. 22.
Fig. 1. "Adolphe Braun: Flower study, Rose of Sharon.” 1854. Photograph. The Metropolitan Museum of Art. Oxford Art Online. Oxford University Press. http://www.oxfordartonline.com/subscriber/article/img/grove/art/F019413. Accessed 21 Sep. 2016.
If you give the full reference in a caption under the picture and you do not cite the work in your essay, then you do not need to include the reference in your works cited page.
All Creative Commons images come with an accompanying licence.
Best practice for giving credit for images you have shared is to include in the caption for the image:
Many digital images with a creative commons licence will have an embed code for the attribution. Copying the embed code is by far the simplest way to include all the details required.
Public Domain images do not require an attribution but it is still good practice to give credit.
If you give credit, it demonstrates to others that you have been careful about the way you have used images as well as recognizing another person's work.
There is no set standard for a credit line but best practice looks something like this:
Image Source: Etude d'une tête humaine, vue de face by Lequeu, Jean Jacques (1757-1825). French National Library (Public Domain).