You need to upload more images

BigStock doesn't sell at the same volume as Shutterstock or iStock. The higher exposure and diversity you have, the more business you're going to attract. With 20 images, you don't have much exposure. I recently started re-uploading my portfolio to BigStock (I resigned about a year ago to become exclusive, then left my exclusivity). I started uploading on April 13th, currently only have 223 images online (am still working on uploading), and I've had 15 downloads. That isn't exceptional, but for BigStock, it's not bad.