Public domain works are no longer under copyright and are freely available to use. You don't need permission to use these images, but it's still good to include some attribution so others know where you found the image.
Creative Commons-licensed works are those where the creators retain copyright, but have automatically granted you permission to reuse as long as you 1) credit the creator, and 2) only use the images as allowed by the license. Some CC licenses specify that you cannot reuse works for commercial purposes, or to modify the images. CC works often have logos like 
or 
, which tell you which license applies to that work. More about the CC licenses.
Along with citing the images as references for your work, images need additional attribution where you use them. For citing Creative Common images:
Figure 1. Image title, by Author. From ref (#), licensed under [CC license].
Figure 1. Bottle of 100 anti-malarial pills, London, England, 1891-194 by Wellcome Images. From ref (#), licensed under CC-BY-4.0.
