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.