Shutterstock actually comes with a very useful tool that I use to keep track of the top 100 keyword searches. This information will help you know what the buyers are looking for and you can create new artwork that are related. Moreover, you can also track the top 50 images of the week and look at the 50 best selling photos/illustrations. Just sign up as a Photo Submitter in Shutterstock and you can access this tool under Tools and Resources>Most Popular Images. Registration is free and easy.