If you are use blogger, please logon to your blog first
Click Template -> Edit HTML
Insert the code below:
<a href="ymsgr:sendIM?wunasaliwu">
<img border=0 src="http://opi.yahoo.com/online?u=wunasaliwu
&amp;m=g&t=1" /> </a>

Please replace wunasaliwu (bold mark) with your yahoo messenger ID, sample : I have yahoo messenger ID : wunasaliwu@yahoo.com, put wunasaliwu only into the code.
And for t=1 (bold mark), it's only display option of yahoo Messenger, you can select one :

t=0

t=1

t=2

t=3

t=4

t=5

Good Luck