I used some img from https://pexels.com to complete sliders for my web & mobile app. Here is its source:

* Photo by Ylanite Koppens from Pexels

* Photo by rawpixel.com from Pexels

* Photo by Evgeny Tchebotarev from Pexels

I also used book mockup from freepik, and the book img from amazon.com