Here’s another nice plugin for working with images in Xcode from Ricky Tan allowing you to automatically creating any missing image sizes called RTImageAssets.

RTImageAssets can automatically detect any missing image sizes and generate your @2x, @1x, and @3x automatically upsampling, and downsampling based on images found. You can configure RTImageAssets to generate only the image sizes you desire.

You can find RTIMageAssets on Github here.