Flickr is an image repository that is artistic in nature. If you're looking for an image that was taken by someone who knows their way around a camera, flickr is a good place to look. Not all of its images are licensed for reuse so be sure to filter by those with a Creative Commons license as show in the images below.
Google Images is likely the most versatile tool one can use while searching for openly licensed images. Though it casts a wide net, Google Images does sometimes return images that are not licensed for reuse even if you have applied such a filter. Best practice is to double check search results to make sure the image you use does in fact have a Creative Commons license.
Noun Project is an icon repository where all of its contents are accessible in two ways: free and not free. Icons can be purchased for use without restrictions or they can be used for free as long as the user abides by the terms of their respective licenses. When opting to download icons for free, Noun Project automatically inserts an attribution in the image. Depending on your use case, this might not be visually appealing. Feel free to crop off that text, but be sure to accompany your use of that icon with language identical to that which you removed in a more visually appropriate location –as you would a citation.