As I wrote an iOS App, which helps you collect the Minifigure Series, that I want to distribute for free and LEGO is not willing to let me use their Minifigure Images, I am looking for the images under a CC or free License.
I don't think that such a set exist. The most complete database are the ones indicated by Guilherme Branco Stracini, BrickLink and BrickSet, but both of them tell in their footer that the content is copyrighter so I am not sure that, especially if you intend to sell your app, any of them will allow you to use their content.
On the other hand, BrickLink provides you with an API that offers a Get-Item-Image so I believe they would allow reusage in some case. You might contact them to get more information, as the API requires a key, they will send one to you or not depending on the expeceted usage.
Last thing and I have never read any clear inforamtion about that, is that Lego and the designers of the company own copyright on the design of the minifigs and that even if the photograph releases a picture under CC license, there are still right that belong to the Lego group so you might not be able to do what you want with these pictures.