Global crowdsourcing platforms could offer new employment opportunities to low-income workers in developing countries. However, the impact to date has been limited because poor communities usually lack access to computers and the Internet. This paper presents mClerk, a new platform for mobile crowdsourcing in developing regions. mClerk sends and receives tasks via SMS, making it accessible to anyone with a low-end mobile phone. However, mClerk is not limited to text: it leverages a little-known protocol to send small images via ordinary SMS, enabling novel distribution of graphical tasks. Via a 5-week deployment in semi-urban India, we demonstrate that mClerk is effective for digitizing local-language documents. Usage of mClerk spread virally from 10 users to 239 users, who digitized over 25,000 words during the study. We discuss the social ecosystem surrounding this usage, and evaluate the potential of mobile crowdsourcing to both deliver and derive value from users in developing regions.