I use images from time to time on my posts. I can insert photos I have taken or insert other images such as clip art or photos taken by others. If I use images from sources other than me, I have to be careful that I don’t break copyright law. I guess I don’t have to be careful. I could break copyright law and hope I never get caught. Even if I did get caught, I would probably just receive a request from the copyright holder to cease and desist or be asked to request formal permission to use the work in question.
I often use Wikimedia Commons as a source of images. Wikimedia holds media that is considered in the public domain. Some of the stuff is usable by anyone at any time. Owners of other stuff require (or hope) that you use it only if you give credit to the source or the maker of the particular item. You can also find video and audio files, some of which are very interesting. Some of the sounds are not particularly appealing but might have utility for specific requirements, such as a bits of noise like this, which, according to Wikimedia, is “10 seconds of blue noise with an RMS value of -12.2 dB.”
You should check out the files on Wikimedia even if you don’t need to use any of them. Many of the images are beautiful, some are really striking, and, of course, some are mundane and should have probably been deleted instead of posted. But who am I to judge? I probably have a different notion of beauty and utility than many other folks.
A Wikimedia “Picture of the Day” is posted daily, and consists of a high resolution photo that has been judged particularly good. You never know what to expect in the Picture of the Day frame.
Of course, Wikimedia has something to do with Wikipedia, that often used and often disparaged encyclopedia of the Web. I use Wikipedia just like millions of others use it. Just be cautious. Anyone, even you and I, can revise an article on Wikipedia. What does that tell you?