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.