Whether or not you register for an account in Artstor, you can search it's large inventory of images. The advantage of creating an account is being able to save search results and revisit them when you wish.
From the Home page, there are two options for searching. One approach is to type a broad, keyword search term in to the search box, and then filter your results. The screen below shows this kind of search in progress:
Notice that the initial search term was Impressionists, and the filters applied were Europe, paintings, and the date range.
Also notice the option to Search within results. When you click it, this allows you to add a search term. You highlight the original search term and type in your second search term. You can customize you search term using what Artstor call Wildcards, which resemble boolean operators. It makes sense to take a few minutes to familiarize yourself with Artstor's Wildcards. You can reach them via the Support tab.
Also from the Home page, you can choose Advanced Search.