as it currently stands, it is not possible to use the Upfront Builder when media files are not stored locally (e.g. in S3 Bucket and Links via CDN). This comes from Upfront resizing certain images and then reuploading them.
While I wish for better S3 integration in Upfront Builder, I believe that is much more to ask of you. So I am asking to be able to disable image resizing and using the original URI for all images when using Upfront Builder.