Besides Wikimedia Commons (already mentioned in @NicolasRaoul's answer, +1), and assuming you're not interested in tongues represented in art or painting (which is not entirely clear from your request), Open-i offers many medical images, including photographs of tongues. Here's an example of a request you can use (caution, these photographs are of medical nature, so viewing discretion is advised): https://openi.nlm.nih.gov/gridquery?q=tongue&it=xg,ph&m=1&n=100. You can refine the keywords as you wish, as well as the filters (license, type of image, etc.).
The US National Cancer Institute also has a search engine where you can look for images or photographs using specific keywords, you can probably find many photographs of tongues there: https://visualsonline.cancer.gov/
Same thing for the US Public Health Image Library: https://phil.cdc.gov/
You'll probably have to classify these images yourself, and make sure you're respecting their license.