Retina ready for uploaded images which are cropped

Sorry I don’t have specific code, but this is how I would do it:

Define two upload sizes, one regular and one double sized for retina using ‘add_image_size’.

Use the filter ‘wp_handle_upload_prefilter’ to add the extension ‘@2x’ to only images of the larger size.

Then use retina .js as normal.

This page as some code using that filter to rename uploaded images, you just need to tailor it to only rename specific uploads: http://wordpressapi.com/2012/01/11/change-uploaded-image-name-to-post-slug-during-upload-using-variables/