The appengine/image package works fine with images stored in Blobstore. However, what would be a good approach to resize images stored in Google Cloud Storage?
You can use the same Image Service with the Google Cloud Storage, especially if you use Blobstore API for uploading images.
A sample code in Java:
String fullName = "/gs/" + bucketName + "/" + objectName;
Image picture = ImagesServiceFactory.makeImageFromFilename(fullName);
Transform resize = ImagesServiceFactory.makeResize(maxWidth, maxHeight);
picture = imagesService.applyTransform(resize, picture);
In Go, you can use BlobKeyForFile function:
BlobKeyForFile returns a BlobKey for a Google Storage file. The filename should be of the form "/gs/bucket_name/object_name".