Gather lots of photos, paintings, frame grabs, etc to help us better imagine your film in a variety of places and time periods.  Reference Materials should be gathered from as many different sources as possible.  If everything you get is taken of the internet chances are you're missing a lot of the good stuff.  Don't be afraid to look through the making of Books in our media library, grab frames from films you have seen, and even go out and take photos of your own.

Broad Reference: At this stage the questions you want to answer are Where and When does your story take place.  Gather research materials based on a number of geographical and historical options that you have available to you.  Is your film set in early 19th century London (Charles Dickens)?  Or a late 21st century space colony (Buck Rogers).  Or is it rooted in 12th century medieval fantasy with a fairytale twist (Shrek)?  This is the development stage where you play with all these options and more. 

Narrow Reference: This is where you begin to gather artifacts that can help to fill out the environments you have chosen with specific details.  Types of hardware, architectural details, types of flowers, etc.  What kinds of things might you find in this environment that would help make it a believable place in the minds of your audience.