AVXCAssets generator submitted by Angel Vasa is an open source tool for automatically generating assets of the proper sizes and placing them within your project.
AVXCAssets generate will automatically scale your icons and other assets converting the images to all the needed ratios, and place them within the Assets.xcassets of your Xcode project.
This animation shows AVXCAssets in action:
You can find AVXCAssets-Generator on Github here .
A nice tool for quickly generating assets of the right sizes within an Xcode project.