ShutterStock (currently the first microstock agency) to enforce U.S. tax laws by witholding contributor earnings by a maximum of 30% for all non US submitters. For US submitters, you will need to submit to Shutterstock a signed W-9.
For more info please login to your Shutterstock account and check your announcements and or forums.
The W-9 form can be found here: http://www.irs.gov/pub/irs-pdf/fw9.pdf