We don't allow plugins on WordPress.com for security reasons. Plugins are a feature only available for self-hosted blogs provided through WordPress.org. You can find out more about this topic here: http://support.wordpress.com/plugins/
However, we do have several built-in features that were inspired by many popular plugins. You might find what you want by searching for it at http://en.support.wordpress.com/
For more information on the differences between WordPress.com and WordPress.org, please see: http://support.wordpress.com/com-vs-org/
If you activate the Sharing features for your site, you can add a Pinterest button per post:
If you wanted to have a button per image, you could try a photography theme, and then have one photo per post:
Please let me know if that is of any help and if you need anything else.