For those complaining about the technical aspects of this proposal, obviously anyone who views the image before the expiration date can save it forever. The point is that after the expiration date, no new people can download the image if they haven't already. Think about your potential employer downloading drunken pictures of your from a frat party 10 years ago. This scheme would prevent that.
Now the fact that this requires a 3rd party plugin to work is problematic. It creates a bottleneck, an extra point of failure, and it suffers from the chicken and egg problem -- nobody will want to post x-pire pictures if their friends can't view them without a plugin, and nobody will install the plugin because they don't need it to see the other 99% of pictures people post.