Have you tried going to Messages/Change my Picture?
From Messages Help.
You can change the picture, or “buddy icon,” that appears next to your messages in the Messages window and in other people’s buddy lists. People can select a different picture to represent you.
iMessage doesn’t represent people with pictures.
Hide
If you’re viewing a buddy list (to display the list, choose Window > Buddies), drag a new picture over your current picture, or click the picture, then select a new one from the list of recent pictures. To use a picture you can edit, do the following:
- Choose Messages > Change My Picture.
- In the Edit Picture window, choose a picture or take a new one.You can use pictures from the following sources:
| Defaults: | Select one of default user pictures in Messages. Click Edit to edit the picture. |
| Recents: | Select a recent picture you’ve taken using the Edit Picture window, or one you imported. Click Edit to edit the picture. |
| Camera: | Take a picture using your computer’s built-in camera. Click Take Picture  to take the picture. After you take a picture, editing controls appear. |
| Other: | Select a picture located on your computer. After you select a picture, editing controls appear. |
- Edit your picture by doing the following:
| Zoom: | Click the bar at bottom of the picture. |
| Pan: | Click and drag the picture. |
| Apply an effect: | Click Effects  , then select an effect. |
Click Done when you finish editing.
https://discussions.apple.com/thread/4661051
https://discussions.apple.com/thread/5156515
