Many images are subject to copyright too. Just because you can access an image on the internet, does not mean you are legally permitted to use that image in your academic work.
Fortunately, there are great sources for non-copyright images that have been released to the public domain, under a creative commons license, or are open access.
Creative Commons image source
The following sources are great places to start looking for images with a Creative Commons license. Even some creative commons images require an attribution message, so make sure you look for that or any other requirements for using the image.
These sites offer at least some images for free general use. These sites also contain advertising and/or offer images for sale. Read each site's conditions for use carefully. Different images from the same site may have different use restrictions, so make sure you know what they are.
Do you have an image, but are unsure of its source?
Tin Eye Reverse Image Search allows you to discover image sources, by searching by image url, uploading the image file, or doing a drag-and drop of the image. You can also search Creative Commons Images by color.