To cite images, you need to include a caption with the image, with ref X where X is the reference number in your bibliography where the image was found, whether it was from a journal, website, or database. If the image is the only item you're citing from an article, you still cite the source in the bibliography.
Captioning images can be a little tricky. Some sites may identify the person or persons who can be credited with the image, while others may not. Please email me (email@example.com) if you have specific questions about citing an image.
While you are not required to include an attribution for a public domain images, it's still a good idea to give credit (including an author if you can find one) if someone else wants to track that down that image.
Image Title, by Creator (if present). Reprinted (or adapted) from ref #. Public Domain.
Figure 1. Nanowires on Lithium Solid-State Battery, by A. Yulaev/NIST. Reprinted from ref 1. Public Domain
Figure 2. Microbial Communities and Soil Carbon Cycling and Storage, by Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. Reprinted from ref 2. Public Domain
The primary difference between citing Creative Commons-licensed images and public domain images it the inclusion of the actual CC license. The creator of the image retains copyright, but has granted automatic permissions for certain levels of reuse. You don't need to request permission, but you still need to give proper credit to that creator.
Image title, by Creator (if present). Reprinted (or adapted) from ref #. Licensed under _______. (see examples below for the how you state the license information)
Figure 1. Ion mobility spectra of HSE (C5H11O7S¯), by Krechmer et al. Adapted from ref 3. Licensed under CC BY 3.0.
Figure 2. Types of biocomposites and examples, by Chang et al. Reprinted from ref 4. Licensed under CC BY NC 3.0.
Figure 3. Microplastics in the Microplastics in the Chesapeake Bay Watershed, by chesbayprogram. Reprinted from ref 5. Licensed under CC BY-NC 2.0.
American Chemical Society grants automatic permission for image reuse for classroom purposes, up to 3 images from 1 article. The image caption should read: Reprinted with permission from Ref (#). Copyright YEAR American Chemical Society. This only applies to images that are credited for the ACS journals (for example, the tables and graphics created by those authors, not images credited to publishers that the ACS authors had permission to use. This does not include Chemical & Engineering News.
Figure 1. Trade in biodiversity loss among 10 world regions and countries, and for the primary pressure categories (total, land use, and GHG emissions), by Wilting et al. Reprinted with permission from ref 5. Copyright 2017 American Chemical Society.