Hi
Very important question you have asked. I had also the same question in mind, at the begining. Sometime we all get confused to use images in right manner for the website.

You can try the bellow link of my tutorial, I think it will help you- Using Images In Website

Like Diane says, SiteContent is literally the best place to start.

I know I missed the adding them in my first few articles in SiteContent because I didn't see where to add them . It's not 100% clear in the training. I think it's slightly out of date.

Here is a screenshot that will hopefully help you

Yes, like Diane said, there are a few you can use including Picjumbo, Pikwizard, gratisography, stockvault, reshot, rawpixel.
For my travel blog where I need unique pictures, I use Reshot and Rawpixel

Hi - yes, you can use Pexels, Pixabay and Unsplash. There are over a million free images here at WA in Site Content you can use.