The payment is on a per-image basis and priced based on the complexity of the brief (if it requires models and/or property releases). The price per image can be found at the top of each brief. For example, if the brief price is $10 USD per image and Unsplash accepts 100 photos from the content you submit in response to that brief, then you would receive $1000 USD. Rates for photos will range, on average, between $5 - $30 an image.
Your images are generally reviewed within 48 hours of being submitted and if accepted, you will receive payment in up to 30 days.