It is very easy if you are using NGUI.
 Create a UITexture.
 Refer it in you class either by declaring it public variable of getting it at runtime.
 Import MiniJSON class that is easily available on internet. (Do not hesitate if you failed to find MiniJSON.Json, it could be MiniJSON.Deserialize or only Json.Deseiralize)
 Following is the code snippet to get Facebook Display Picture with User_API_ID