If you have a WordPress.com hosted blog and want to add LinkedIn as another available service for sharing just follow these steps:
1) Upload a 16x16px icon you want to use to a photosharing site or your own Web site. Need an image? Find some images to use here.
3) If you aren’t logged in, log in
4) Got to the Settings panel and click Sharing
5) In the Available Services box click add a new service
For label enter: LinkedIn
For URL enter: http://www.linkedin.com/shareArticle?summary=%post_excerpt%&title=%post_title%&mini=true&url=%post_url%&source=%post_full_url%
For Icon enter: The URL to the image hosted elsewhere