To help identify your user account from others who use your system, OS X assigns your account a small picture that appears in various places in the system, including the log-in window as well as in applications like Mail.
When you initially set up your account in OS X, the system will have you set up your account picture, but often people skip by this step and continue with the setup process to start using their computers quicker, leaving their accounts with the image that OS X randomly chooses for it. However, if you would like to change your account picture to something other than its default icon, then you can do so in the Users & Groups system preferences.
Go to the system preferences and click Users & Groups, where you should see your current account listed and selected. In the main "Password" tab of these preferences you will see your account picture, which you can change in several different ways. The first is to click it, where you will see a small menu appear below the window. In this menu the built-in account pictures of bugs, pets, wild animals, and flowers will show. There is also a "Recents" section where both these and other pictures you've used for your account will be listed.
In addition to these, you can click the "Camera" section to take a picture of yourself using your Mac's iSight or other Webcam device.
While these options and services are part of the built-in OS X images you can use, if you would like you can also use any image of your choosing. Simply drag the image from your desktop, iPhoto, or even a Web browser to the account picture.
After you have the image of your choice, the next step is to edit it to your liking. To do this, select it in the "Recents" or "Defaults" section of the account image browser and click the edit button, which will appear as a small styled pencil above a background rectangle. In the editor you can zoom in or out and reposition the image. You can then click the effects button, which looks like a little swirl next to the zoom controls, where you can apply a number of effects to twist and warp the picture, invert or enhance, or otherwise filter it.
These options can be performed not only on your account, but to any other account that is not currently logged in (if you have administrative privileges). To do this, click the lock at the bottom of the system preferences, which will allow you to select and perform the same actions on an account once you provide an administrative username and password.