You can get the user's ID from the returned response, then use the below to get the profile picture:
http://graph.facebook.com/User_ID/picture
I use SDWebImage for images:
NSString *urlstr = @"http://graph.facebook.com/100001393655684/picture";
[imageView setImageWithURL:[NSURL URLWithString:urlstr]
placeholderImage:nil];
For Twitter, please make sure you authenticate your request, check this link for more details
Note: You may need to update TWRequest to SLRequest if you aren't deploying for iOS5