Editor dropdown with a list of predefined links

[Meorazhar]

I run a forum where users upload copyrighted pictures from a number of other sites. There is a requirement that they should put a link to the originating sites in their posts but posters barely do that.

I thought showing a dropdown of, say, 30 of the most popular sites in the editor, somewhere close to "insert link" button could help. A user picks a site name from the dropdown list and a certain code is inserted into editor window, for example:

The pictures are courtesy of http://site_address

How do i do that? Perhaps something like that has already been implemented, pls guide me in the right direction.