I have a number of problems with this proposal.
1) Implementing will almost certainly violate my ‘no major changes’ policy for this plugin. If I were to implement this, it would be in a new plug-in that makes use of modern WordPress’ custom fields options to store credit data in.
2) Not every user limits themselves to a single image source. There are many other liberal licenses available, and offering them all through a drop-down menu would make the menu unwieldy.
3) Copying and pasting a license URL means you just have to do that. There is fairly little chance of error, unless you have been copying different URLs for different licenses. Selecting a license from a drop down however means you have to be able to identify the license names, which often consist of similar sounding legalese. I am not sure a drop-down is the safer option.
4) Having a text field allows for out of the ordinary things like photographers writing their own licenses.
Having said that, I am open to suggestions and arguments.
One way to implement this to only offer the three or so most recent license URLs through a drop-down menu, and keep the text field for everything else.
One way to avoid the dropdown problem (having to choose between cryptic sounding license names) might me to couple the license choices to the image source. The plugin could determine from the image URL what the available license or license options are.
The photo service should probably provide an API for this; if one were to build a scraper, the scraper could break down every time Flickr changed its lay-out.
Flickr does have an API, but does not seem to support requesting the license of individual images. (http://www.flickr.com/services/api/)
Wikimedia Commons does support requesting license info: http://commons.wikimedia.org/wiki/Commons:API
Important here is to note that the response is not always complete, at which point the plug-in should make a choice: show all the licenses? show only the ones with complete info? show none and an error message?
