<div align="center"><a href="ymsgr:sendIM?YAHOOID"><img border="0" src="http://opi.yahoo.com/online?u=YAHOOID&m=g&t=14"/></a></div>
just replace the "YAHOOID" with your yahoo massenger id. and you can also change the image as you desire but there are only 20 images that you can use one of them.
Now, to find the image you will needto copy the following code to the browser address field and then enter.
http://opi.yahoo.com/online?u=hell0172&m=g&t=1
http://opi.yahoo.com/online?u=hell0172&m=g&t=2
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http://opi.yahoo.com/online?u=hell0172&m=g&t=20
After selecting the perfect image for your online and ofline image, go to blogger.com and sign in with your account then go to layout section ad Java and html gadget. Paste the code in this gadget and save it
Enjoy