I host my image website to Amazon S3, and want keep direct access for everyone. When i upload new file to my S3 bucket, i must change the files acl manual. Is any idea to change the default acl for a bucket?
ps. i used the S3Fox tools under MacOSX.
It is not possible to change the default ACL, which has been the first access control mechanism available for S3.
However, nowadays you can (and probably should for anything except the simplest use cases) use IAM Policies and/or Bucket Policies instead of or in addition to ACL's, see Using IAM Policies for a comparison of all three mechanisms. Please ensure you are aware of the effects of using IAM and Bucket Policies as well as ACLs and Bucket Policies together though.
A good summary is provided in Amazon S3 Bucket Public Access Considerations, with a specific section Hosting Website from Amazon S3 and Bucket Permissions addressing your use case.