Below are several very thorough lists put together by other libraries, and some of the resources listed may be subscription databases not available to students from other universities.
These sites free-to-use images. Please make sure that you understand the licensing on an image before downloading it as licenses vary from site to site. One of your best bets is to look for images with a Creative Commons CC0 license since you can treat works with a CC0 license more or less the same way you would treat a work that has passed into the public domain (i.e., basically use it however you'd like).
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