Skin
Which one did you find easiest to use or best suited the kind of image you were looking for? I used google search engine, with the key word skin. From there I clicked on the “images” icon, then the tools tab clicked on the usage rights, and selected creative commons licenses. I selected the image I felt would meet the need and then clicked on the image. From there I downloaded it and was provided with the source, author, and link to be used for citation.
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Example for "Skin":
https://commons.wikimedia.org/wiki/File:501_Structure_of_the_skin.jpg