Just think: most of what the library gives you access to is words--books, articles, databases. They're full of words, and you use words to locate what you need from that collection of words. Images (and visual media in general) often don't work the same way. They might be described using words (known as metadata, which can consist of the title, creator, technical specifications, or other information about the information), but generally those are going to be more limited in scope than, say, searching the contents of an entire book to decide whether it's relevant.
And yet, most collections of images ask you to use keywords to locate items.
So to find images, it's a good idea to switch up the approach and be aware of the way the images are described and organized just as much as you're thinking about what types of images you're looking for.
While there aren't a lot of rules that apply to every single image collection, there are a few that are broadly applicable:
The following are a selection of image collections, libraries, and archives that are generally intended to be used for study, though some may be in the public domain or licensed for reuse. For images explicitly licensed for reuse and modification, check the next tab.
Note that many image databases linked elsewhere might include open access and/or public domain images and often are able to be filtered accordingly. The following are resources that primarily feature images licensed for reuse and/or that are in the public domain.
Be sure to check in the general art image databases for design-related content, too!