Using different files for the same project on different devices in iOS?

Say if I have 3 animation files for a size for iPhone 5s, iPhone 6 and iPhone 6 Plus. What about ipad?

For the same view controller, how should I use these respective files for different iPhone sizes. Here's a specific example:

NSString *filePath = [[NSBundle mainBundle] pathForResource:@”iphone5s” ofType:@”gif”];

but what if I want to load for iPhone 6, iPhone 6 plus called iPhone6.gif, iPhone6Plus.gif etc

If you know, please let me know in Objective-C.