As of this post, cocos2d-iphone requires both retina and regular file for retina mode. We cannot supply only retina images.
Here is a Guardfile for generating regular (standard, non-retina) images on the fly.
When there is a new/updated image ending with -hd.png in the folder, a non-retina version of that image is created/overwritten by imagemagick. This isn’t the best solution for creating low-res images, but it is a fast fallback solution.
Although this script resizes PNG images with the suffix -hd.png. Images with other suffixes or file formats are supported by modifying the script.