From the API Guidelines:
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​When displaying a photo from Unsplash, your application must attribute Unsplash, the Unsplash photographer, and contain a link back to their Unsplash profile. All links back to Unsplash should use utm parameters in the form of ?utm_source=your_app_name&utm_medium=referral 

Attribution is a key part of the Unsplash ecosystem as photographers rely on the exposure gained from their work being seen by millions of people.

While the Unsplash License doesn’t require attribution because some uses may make it hard or impossible to attribute in a meaningful way, all of the API uses we’ve seen can easily attribute in a clear and beneficial way to the photographer.

When attributing the use of a photo, we recommend taking the form of:

Photo by Annie Spratt on Unsplash

In HTML that would look like: