@laurensmartina implemented this in #344, but we discovered that the longdesc attribute is supposed to contain a URL pointing to a description of the image (similar to the alt attribute, but longer). See https://www.w3.org/TR/html-longdesc/#UCnR

So using longdesc here is not quite ideal. There does not seem to be an HTML standard attribute for attribution/copyright/license, so maybe just data-attribution would be better.

As for displaying it, I wonder if displaying the attribution info as a caption makes sense, I'd be more inclined to show the title tag as a caption maybe. Looking around, it seems the <small> tag (meant for the "small print" in HTML5) might be a better fit, see: https://stackoverflow.com/a/21489636/740048

Additional improvements (as indirectly suggested in the Stadsbron marketing advice) would be to offer more guidance to the users about alt tags, to encourage them to enter a meaningful alt tag (probably also stop autofilling the alt tag with the filename).