Manage images manually
Your best option for remote images is the Filefield Sources module, however this will not provide the ability to manually create the thumbnail, medium, large, etc versions.  

You can achieve auto-creation with image-styles and preset the dimensions or criteria that you would like for the "cropped/smaller" versions.

Lastly, if you want manual creation of those files you would have to create a new field for each version and have them pre-cropped/sized, and paste in the url from the CDN.