The best way of finding images you can reuse without separate permission is to use the advanced Google image search engine at https://www.google.com/advanced_image_search.
You can use the drop-down at the bottom to choose the licence option 'free to use, share, modify even commercially'.
This looks for Creative Commons Attribution-licensed images and others on an open license. If you limit your search this way in the beginning, you can find images quickly which you can attribute to the creator in your work, without having to contact the creator for any separate permissions.
You can see an example of how to attribute Creative Commons-licensed works at https://creativecommons.org/use-remix/attribution/.
For further guidance and other image banks to try see our Images Guide.