Here's how to create a custom picture for your OS X profile, as well as how to update your pic in Messages.
Whether you have a photo that is already on your computer, wish to select one of those included with OS X, or want to take a new picture, you can do so in System Preferences.
How to do it:
1. Open Apple menu and System Preferences.
2. Open the Users and Groups preference pane.
3. Select your account from the list on the left.
4. Click the "Password" tab to the right.
5. Click the user profile photo that is currently set.
In the popover dialog that appears, you can choose from the many pictures provided with OS X in the "Default" category, which are perfectly sized to fit the allocated space.
To use a custom picture, drag and drop it from the Finder onto the slot that contains your current picture. Creating a new picture is also easy as pie. choose "Camera" on the left to see the view from your Mac's camera and take a photo of yourself.
This will not change your photo in Messages, however. To change that, you'll need to do the following:
1. Open Messages and select Window and Buddies.
2. In the list that appears, select your profile photo, and then select "Recents."
3. Select the photo that you applied to your account in System Preferences, and then click "Done."