Describe It! is a Zooniverse project of the Frick Collection and Frick Art Reference Library that seeks to crowdsource alt text descriptions for the collections 1.2 million digital images. Volunteers are prompted to write descriptions of fifteen words or less after viewing a brief tutorial that walks them through the process.
Each image is described three times before it is retired, at which point library research staff select and append the best alt text description. Volunteers can participate anonymously or create an account.
Describe It! provides a useful model should you wish to collect alt text descriptions via crowdsourcing. The project tutorial also suggests one way to describe alt text to users.