DISPLAY IMAGE
If you mean the display picture that shows at the top of Yahoo Messenger, and in IM windows then go to the top of Yahoo Messenger and choose the Messenger menu. Click My Display Image. If you want to Share you Yahoo Avatar or create one then tick that option. Click customize if you want to make one or to change yours. You can also change it by clicking the area where you display image will show in the IM window.
If you want to just share a different picture then you would have to have one on your hard drive. You would choose the options, "Share my picture," and then click Select. You will then either choose one of the default ones yahoo provides or browse for the one you want to use.
Or you could also put a photo on your Yahoo! Profile. Read this tutorial to learn how to change your Yahoo! Profile photo:
http://help.yahoo.com/tutorials/prof/prof/prof_photo1.html
