1.go to wordpress.com

2.Click appearence

3.In that widget

4.Add text widget

5.past the below code

(same you can add external embed code)
<p><a href=”–Place your linkedinURL–” rel=”nofollow”><img src=”–Past yoru ImageURL–” height=”40″ width=”170″ style=”border:0;” alt=”" /></a></p>

6.Thats it

Thanks,

vinod.D