The best one for searching the free images is still Google. Google Image Search has a "Advanced Image Search" where you can choose the usage rights. Choose the "free to use, share, or modify, even commercially" option will lead you to the most free images. I have prepared a link for you with the usage right selected:
Google Image Search with free usage right
Another 2 good images providers:
Pixabay - lots of good free high quality images.
Freepic - lots good vector images, suitable for slides or posters.
Note: double check each image's license before you use it.