To use the Flickr API you need to have an application key. We use this to track API usage.
Currently, commercial use of the API is allowed only with prior permission. Requests for API keys intended for commercial use are reviewed by staff. If your project is personal, artistic, free or otherwise non-commercial please don't request a commercial key. If your project is commercial, please provide sufficient detail to help us decide. Thanks!
Apply for your key online now, and please make sure you include a brief explanation of how you're planning to use it.