For non-unique graphics requirements, stock images are a common option to consider. There are both free and commercial stock images of various categories and qualities available on the Web. Some are free for both private and commercial use, while license needs to be purchased to use some stock images commercially or for specific purposes. A license may be for unlimited use (as in the case of royalty-free stock images) or for just one-time use (as in the case of rights-managed stock images), or there might be other terms involved.
Source: Sklar David (2016), HTML: A Gentle Introduction to the Web’s Most Popular Language, O’Reilly Media; 1st edition.