To change your display image, first, you'll need the desktop version of Yahoo Messenger. You'll need to download and install it before you can change your display image.
Go to the help article "How Do I Select a Photo to Display to Others?" to learn how to change your photo.
To change your avatar, visit http://in.avatars.yahoo.com/. Sign in and customize your avatar. Your changes stay in effect until the next time you make a change.
Go to the help article "How Do I Display or Hide My Image?" to learn more about configuring you display image.