Here's the first of three scrapbooks (link is to the Flickr set) I made when I was writing Sea Hearts/The Brides of Rollrock Island.
Here's the second scrapbook.
And here's the third.
And here's all three of them in the one set.
Updated: Wendy Orr mentions the scrapbooks, and talks about visual aides-memoires, over at her blog.
But the best images of all are the ones that capture the mood you're looking for for this story, that lead you into the emotions that your characters are feeling. It can be the colours of a sunset, the expression on a face, the flip of a pony tail... there are no rules. You'll know when you find the images of the clues that'll help you dive deeper and focus more sharply in the world you've created.