As explained in the Site Overview, you can search for a photograph according to topic or theme.
There is a more systematic way, and that is to use the search box located at the top right of a typical screen.
By entering a word or text, and clicking the magnifying glass, a search for keywords is performed.
The results summarize the number of Galleries, Collections, and Photos satisfying the keyword in tabs. By clicking the relevant tab, you can access the actual photos.