In order to access the image from the Asset Catalog, you only need to access the name of the asset group without any extensions.
So, if you add an image named @"my-button@2x.png" to the Asset Catalog, it will create an asset group called my-button.
Now, all you have to do is access the image like so:
// Objective-C
[UIImage imageNamed:@"my-button"];
// Swift
UIImage(named: "my-button")
Also, you can edit the asset group by renaming it (without renaming the images) or changing it’s individual components. This will allow you to follow easier naming conventions as well as show completely different assets between different UIScreen scales without any scale checks.
In order to incorporate images for different device sizes, you may need to toggle it under the “Devices” subheading in the Asset Catalog Group’s options. Here is an example of that toggle (available by right clicking the group).