A large community of mobile photographers use Twenty20 (earlier known as Instacanvas) to explore photography and earn cash from their own photos. Nearly 300,000 photographers and 46 million photos give brands and digital creators a lot to choose from.
Use the free Twenty20 iPhone app to join as a photographer. Those without an iPhone can use the website as there is no Android app as yet.
Twenty20 has a good explanation of the type of photos that get sold.