I found this in the FAQ section of Steemit. I think it would be extremely helpful for many steemians.

"Is it okay to use random pictures from the internet?

If you are using an image that is not your own, make sure you are allowed to use the image, and cite the source of the image.

Using random pictures from the internet without giving credit is discouraged. You may, however, use photos from “free image” websites such as Pexels.com or Pixabay.com. All photos on Pexels and Pixabay are free for personal and commercial use.

Here is a post from @mindover that has links to many websites that have images you can use:
https://steemit.com/steem-help/@mindover/don-t-plagiarize-images-here-are-13-free-and-legal-ways-to-find-high-quality-photos-you-can-use-on-steemit

So, i guess using free images from pexels and pixabay are okay.