You may not be aware of this, but some hosted on the Amazon web servers have expiration date. These dates are more commonly seen on Tumblr, but any image that come from these servers may have been deleted already. How to know this?
Look at the “Expires=1316556799”, no, that’s not a date we can read, it’s a date in Unix time that you can read using this service, and will show a result like this: “Tue, 20 Sep 2011 22:13:19 GMT”.
Of course, not everything is lost, the user uses a bot that will turn any image submitted in reddit that expires into a imgur link. The original image is this. The explanation:
Tumblr stores their images in Amazon S3. As Amazon charges Tumblr for bandwidth, they’ve apparently decided to only allow authenticated requests, rather than open it up for anonymous access. Tumblr can provide temporary access to these images so your browser can download them by signing the request with an expiry. So, the first URL just generates the signed URL, then points your browser at it.
And that’s why if you don’t clean your cache, you’ll be seeing the image forever, until you clean the cache. You can easily test this with this image that will expire “Fri, 23 Sep 2011 16:42:02 GMT”.