Pixabay is a decent site for images, but also very limited. Shopify also has a good selection of royalty free images that people can use: https://burst.shopify.com/free-images
For those who want more professional looking images try Fotolia. The platform is owned by Adobe, and it sells a huge selection of high quality photos for fairly cheap prices: https://us.fotolia.com/
As for plagiarism, this seems to be a big issue on YouTube as well. People often steal videos and re-upload them to their own channels and then dump ads on them. Its the whole reason Google keeps having to change the rules regarding monetization, because a few bad actors are stealing content left and right.
One of the things people can do to protect their videos is make use of a logo to include in each video production. I do this for the majority of my gaming videos, and include the logos for my gaming brand in my videos here on Steemit. This way if someone is stealing your content, others will know because of the logo, and someone will always tip you off.