There are many apps to resize retina images in the Mac App Store. The best one I've found is called Sizes. It needs me only to drag my images and drop them on the icon at the dock. And that's it. No distractive windows, no inappropriate messages. I don't even need to name files properly, because Sizes does it for me automatically. For example, if I have a file example.png and it is 200x200, then Sizes will create 2 files: firstname.lastname@example.org which is 200x200 and example.png which is 100x100. Really simple.
There are also some options to have more controll on this app. Here is how it looks like:
More information and download like on the product website: http://www.apptorium.com/products/sizes. Really recommended.