When you're searching with Viewfinder, you have a number of options:
You can search the title, tags and description of a photo, or just the tags by selecting the "All Text" or "Tags Only" menu item from the pop-down menu in the search field.
You can sort the search results by 'relevance', 'most interesting' or 'most recent' using the filter bar.
Finally, you can also hide images whose largest available size is below a certain threshold. Select one of the options in the "Min. Size" section of the filter bar, and photos whose shortest side is below the selected minimum size will be hidden from your browser.
Viewfinder allows you to limit the scope of your search to only return photos which are licensed with Creative Commons licenses. When you click "Creative Commons" in the "Rights" section of the filter bar, your search will only return photos which have a Creative Commons license.
You can further restrict your search to only return photos which have a Creative Commons license allowing either commercial use or the creation of derivative works, or both. You can add these restrictions by clicking on "Commercial Use" or "Derivative Works" in the "Rights" section of the filter bar.