You're right, it's possible but you must import directly your storage
and do not rely on django.core.files.storages.default_storage in this
case. This way you can initialize directly your storage with your
bucket name and pass the storage to the right field. Untested pseudo-
code:
s3storage = S3Storage(bucket="your-bucket-name", etc)
class YourModel:
yourimage = ImageField(storage=s3storage, etc)
Hope it helps,
David