The most important tip I can give to new users is to make sure that their posts have great images in them. A close second is to make sure that you document your where you got them from and ask permission first if needed.
I have 3 go to sources that I wanted to share with you whether you are a beginner or a slightly more experienced user. They all have 2 things in common ... 1) they are FREE & 2) they are GREAT
The first is the PIXABAY https://pixabay.com
The Second is Visual Hunt https://visualhunt.com
The 3rd and final free picture site is https://unsplash.com
These are my favourites... let me know if you have any other that I missed please.
Image Sources: Pixabay and Websites listed