The Galleries are theme based, and can be grouped by subject, for example 'Food' or 'Travel', to make browsing by category easy. Photo and Illustration/Vector galleries are also separate, for easy browsing by file type. However, if you have a specific subject in mind, the Search feature allows you to quickly focus on what meets your criteria.
Use a keyword to help narrow things down. Be specific, and use only one keyword, as the search function does not search on multiple keywords.