Go to your dashboard, and under the chosen blog, select layout
Under Add and arrange page elements, select one of the two add a gadget depending if you want it on your sidebar, or at the bottom.
Give it a title.
In content type a html img tag containing the following
replacing setitesuk with your username or nickname (male can be switched to female).
click save and hey presto, it will now appear the next time you load your blog.
Note: I am sure I am duplicating this post from somewhere. Partly, this is for my own info, partly to help anyone who also can't find the details easily (apart from mst's blog about the url for the image.)