Christoph is right. This has been available since the first day that Photosynth let you upload your own photos: 2008 August 20.
One thing to note is that many sites strip all but the most bacic HTML, such as links, out your post. Facebook does this, as does Wordpress.com and even Microsoft's own Live Spaces. If you're running a Wordpress installation on your own server, you can choose to allow HTML to be posted and all will be well and in Facebook's case there is an unofficial Facebook app, MySynths, which allows you to add a custum thumbnail and description to each synth that you post on Facebook.
MySynths is a bit too manual, but it gets the job done.