If I understand, you need to specify your domain name instead of S3. To do this, you need to write a handler which will act as a proxy to the amazon server.
But, that would actually double your bandwidth as you need to send the image to the client as well fetch the image for S3.
I am not sure is there any S3 specific way to handle this.
Below is Amazon S3 specific way taken from Amazon S3, CNAME record
The average user may sign up for
Amazon S3 hosting and begin uploading
files - accepting the default URL
structure for hosted files:
http://s3.amazonaws.com/your-bucket/some-file.txt
If you plan on using S3 to host media
files - more specifically Flash files
- you’ll run into Adobe’s cross domain security policy. The fix requires
mapping your hosted S3 files to look
as though they are being served from
your own domain - virtual hosting. The
easiest and most attractive method
would be a hosted file URL that like
this:
http://s3.your-site.com/some-file.txt
To get started, create a bucket on S3
that you want as the root for your
hosted files. For this example, your
S3 bucket would be:
s3.your-site.com The most important
step is adding the appropriate CNAME
record to your DNS settings.
Name Type Data
s3.your-site.com CNAME s3.amazonaws.com.
Expect your new DNS settings to take
up to 24-48 hours to resolve.