Because the Web has grown up in a commerical age, we tend to think of databases as a kind of shopping platform, a place to find and consume texts. But scholars rely on tracing the conversation through references, using the online platforms mainly to locate things they have heard about. Conversation is much better metaphor for research than shopping is. A philosopher put it this way:
"As civilized human beings, we are the inheritors, neither of an inquiry about ourselves and the world, nor of an accumulating body of information, but of a conversation, begun in the primeval forests and extended and made more articulate in the course of centuries. It is a conversation which goes on both in public and within each of ourselves. Of course there is argument and inquiry and information, but wherever these are profitable they are to be recognized as passages in this conversation, and perhaps they are not the most captivating of the passages . . .
Education, properly speaking, is an initiation into the skill and partnership of this conversation in which we learn to recognize the voices, to distinguish the proper occassions of utterance, and in which we acquire the intellectual and moral habits appropriate to conversation. And it is this conversation which, in the end, gives place and character to every human activity and utterance."
Michael Oakeshott. The Voice of Poetry in the Conversation of Mankind: An Essay. London: Bowes & Bowes, 1959, 11.
The thing about conversations is that not everybody is made welcome. As you do your research, think about what voices and perspectives are missing.
Think, too, about how all forms of organizing information have biases. The library's classification system for books is based on a view of history that treats social history separately from national histories and great wars. Subject headings used in cataloging reflect biases: as an example, many librarians urged the Library of Congress to stop using "illegal aliens" as a subject heading and instead use "undocumented immigrants." Though the library was willing, Congress was not and for the first time legislators decided what subject heading would be used in libraries: illegal aliens. Language is political.
Archives and museums may tell people's stories differently than people themselves would or may not bother to keep things that are deemed "insignificant." The algorithms that drive search engines aren't free of bias, either, and the way the biggest internet corporations use personal information to shape your search results (and advertisements) distorts what you see.
There are at least three modes of search practiced by historians.
Strategic Serendipity - putting yourself in a place where something interesting may turn up.
Deliberate Inquiry - searching databases, refining search terms and parameters as you go
Tracing conversations - using footnotes to trace ideas both forward and backward in time.
You're likely to use all three of these modes throughout your research. Think of it as a cycle - at first you may not be sure what you're looking for, but as you dig and explore, you learn more and your question gets sharpened, more focused. You may find out things are far more complicated than you realized or you may find a primary source that throws up new questions. It may feel like you're going in circles, but that's how research works.
Browsing shelves and browsing collections of primary sources can seem aimless, but if you are thinking actively about your research question, you might stumble across interesting material that raises new questions, provides evidence for a theory, or offers a new avenue of exploration or a connection you hadn't seen before.
Some examples of sites for browsing historical images
Digital Public Library of America - curated collection of digital materials from cultural institutions across the United States
Flickr: The Commons - images from cultural institutions with no known copyright restrictions
New York Public Library Digital Collections - search using specific terms or browse thematic collections of photos, videos, images...
Books are shelved in general subject categories using the Library of Congress classification system. You may want to supplement your use of the catalog with browsing shelf areas for your topic. Below is a brief listing of some of the subject locations in the field of history.
Sometimes you have to get some context or find out the basics of a subject without going into great depth. Of course, there's Wikipedia, but if that doesn't give you the insights you want, try a specialized encyclopedia. These are good for getting a brief but expert explanation of a topic, a biography, or an overview of something tangential to your research. Reference books may also point you toward a small number of influential sources. Here are some examples - but a reference librarian can guide you to more.
Encyclopedia of American Social History by 
Brill's New Pauly by 
A Native American Encyclopedia by 