If you use photos you find on the Internet in your social media posts, make doubly sure that you have permission to do so! If you do not, you could receive a bill in the mail for royalties, and that bill could be for thousands of dollars. That is a mistake that lots of people have made, and it can really sting! So just be careful.
However, there are some sites that are a repository of royalty free images. What you are looking for are images that are listed as (1) free for commercial use, and preferably (2) no attribution required. The ‘no attribution’ part just means that you don’t have to give them credit for using the image in a social media post.
There is a large number of sites to peruse for images. Below are a few:
Note that some of these sites also sell images, so if you’re looking for free ones, be sure you search the free sections.