Take note, content on pixabay is available under the Pixabay License. It allows commercial use without attribution, but be aware of the terms and note that it is somewhat less permissive than CC0.
Edit: @LiliArch raised an important point. Check the individual license of the each image you intend to download from the site.
If you need more sources, you can refer to this post. Also add Rawpixel and PublicDomainPictures.net to the list.
I know Swamp Castle used stock images with little regard for consistency, but I actually found it quite likeable. 
