I would second the mention of "redundancy" -- you can count on any content that's in S3 to be distributed to multiple data centers, and effectively been very much always accessible for anyone with functioning network connection.
Cost may be another factor: data transfer rates for S3 are quite competitive.
And speed is the last one: you can access data VERY fast from S3. But that's more of an issue for data other than browser-viewable images.
