This is the most simple and it will be a short one.

Sites from where you can download free full rights images:

• https://unsplash.com
• https://pixabay.com
• https://www.pexels.com
• https://gottapics.com/gallery/

Here you can get more detailed information:

https://wealthyaffiliateworks.com/the-best-websites-to-get-f...

I'll asume you have now the photos for your blog, first step!

Let us continue now with step two - goes a little bit more complex, but nothing to worry about. Once you did it once, you'll master it. It is really so simple ;)