Beginning with your artwork and your questions, identify some of the topics you'll be looking for. Use any additional information you found about your piece (such as the description on a museum website, or information from the Artstor database) to help specify the type of information you'll want to find.
Taking the above piece, Stag at Sharkey's, we may have asked any number of questions that have to do with boxing. From the description on the museum website, we learn that the setting for this painting was a private athletic club in New York City. Since we're told that the artist visited this club himself, we can assume that the time period is somewhere around 1909, when the painting was created. We also learn that public boxing was illegal in New York at that time.
At this point, you may have already tried to search for something like New York boxing history and come up empty. In order to be successful, a search like this essentially requires that a highly specific source exist. So, where should we start now?
Since we don't know much about our topic of boxing in early 20th century New York, we'll need to try something more broad. Academic subject encyclopedias and books are good sources for broad overviews of a topic. Finding an encyclopedia entry or a book on the history of boxing, or another topic related to boxing, might be able to fill in some of the information we're looking for.
Before searching in the databases, you need to generate keywords from your research questions. Search engines don't know what you're really looking for - they just search for the words you type in. Choosing good keywords is the first step towards a more successful search.
Let's look at the example of Vermeer's The Milkmaid, featured in the Berger article.
From your reference research, you should now know a few things about your painting that can help you investigate these questions further.
Keywords for Question #1
Keywords for Question #2