In our Joomla website we'll likely be publishing a large quantity of images under license from advertisers, government agencies and third parties.

Is there an extension that allows the image authors and/or licensors to upload their images, specify the conditions of use, and sign a license agreement with the website?

I would imagine that they would have to do so through the front end, with author privileges; then the administrator would decide which images to use and authorize publication in the site.