You can find free mermaid images on Stockfreeimages.com, 123rf.com and Shutterstock.com. As with most stock image websites, you can download a limited amount of images or small ones for free, with the option to pay for larger sizes or quantities.
All three of these sites offer a variety of different kinds of mermaid images. There are drawings or paintings of mermaids, cartoon mermaids, real people dressed up as mermaids, photo-shopped pictures of women made to look like mermaids and pictures of real women who somehow resemble mermaids. For a few dollars on any of these sites, you can get unlimited access to mermaid images, as of 2015.