Using photos from likes to use in application
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I am in the process of creating a Facebook Application, specifically a social game. The game requires that users upload a photo of their choosing. However, if the user doesn't have a photo they wish to upload, they would select one from a stock of photos. I am wondering if it is possible to allow users to select a photo based on the pages/activities/etc that they "Like". So for example, the app can be coded to have a selection of photos based on a users "Likes". If the user has "Dogs" as a like, then the app will automatically find pages on Facebook that have images of those likes and those could then be used in the game. 

My questions in regard to this idea are: 
1). Is this legal to do from a copyright aspect?

2). Would it just be easier to go on a site like Shutterstock, buy the rights to photos and manually create the database of photos that way?

3). Is it possible (design/coding wise) to create the method of finding relevant photos that I just listed here? 