Finally ,I got the key:
OW don't allow the S3 bucket name to be images.mydomain.com for example, the name can't contain "." which is allowed by S3 bucket naming rules.
However,we need to set S3 bucket name to be a subdomain to costomize S3 urls for many advantages,in particularly,we can't use cloudfornt without subdomain name I guess.
So,I hope OW could support subdomain naming for S3 bucket name.
Regards,
Xavi