A lot of people use these free stock photos so you might see the same image on someone else's website, but don't worry, the internet is big.
We also recommend you take the time to customize or personalize a picture. If you know how to use photoshop, it's perfect. But if you don’t use photoshop, don’t worry, there are free alternatives to photoshop such as Gimp or Pixlr. Relax, if you don't know how to use these editing softwares, you can use the Photoshop Online Tools to edit photos or Canva to create nice designs. Canva is free and really easy to use. You don't have a to be a great designer to do something pretty and it's actually a lot of fun. Canva is definitely one of my favorites!
When you pick a picture, take your time and think it through. Some say that people will be more likely to read and remember your content if it contains colored images. It's probably true since searchers say so (and I am personally instinctively attracted to colorful images) but the relevancy of the picture is significant as well. A content with a relevant image get 94% more views than one without a relevant image! Illustrate your point but also target your audience.
It's up to you know to choose the right pictures! Remember that cheesy saying: «a picture is worth a thousand words»? It has never been so true!