Deana Valorose knows that each click of the camera on her smartphone could mean money in her bank account.

Valorose, of Hollywood, Fla., said she's made more than $300 from selling 68 of her pictures - from breakfast to sports and nature - through a free app called Foap.

The app sells the photographs to big brands like MasterCard for use in their advertisements.

Each photograph on Foap sells for $10. The owner gets $5 and Foap gets $5.

Related: Get more information on selling stock photos

"The picture that I've sold the most of is parasailing - people parasailing," Valorose said.

Business is booming, according to Foap, because it's much cheaper for companies to buy stock photos from sites like Foap than from the professionals.

Foap said that half of its users were making money and that half were raking it in.

The app's user base has grown 25 percent in the last financial quarter as the company seeks to win a share of the $4 billion market for stock photos, competing against longtime stakeholders iStock, Getty and The Associated Press.

Duyum Dulom of California made almost $4,000 in less than a year by uploading his photos and winning competitions on Foap. Sony and MasterCard each have bought his images. And, he said, Heineken paid him $100 for an image.

Some big brands have even started sending users out on specific missions. MasterCard paid Foap user Adam Hamilton, 42, of Portland, Maine, $500 for this picture of his son playing in the snow.

Sneaker and apparel brand Puma paid Hamilton $2,000 for an image.

At first, Valorose said, she was skeptical about selling her images.

"Once I sold a couple pictures, I went to cash out," she said, "and got my money. … This is the real deal. Yep!"

ABC News' Michael Koenigs contributed to this story.

Photo credit: Deana Valorose