Other than in Windows, the account picture in Ubuntu and Mint isn't displayed in the login-screen but everytime you enter your password for authentication as well as in the user selection applet in Ubuntu.
To change it, open the main menu and type in user accounts. On the window that pops-up upon opening the application, click on the head-icon on the right and select a new picture from the list or browse for more pictures.
Apart from the traditional text string password, Windows 8 offers a new form of securing your user account which is by including touch and/or gestures into the procedure. Spoken directly, you are now able to select a picture where you perform three mouse or touch gestures on (circles, line or dots) that will unlock your account if repeated correctly.
To create such a password open the Charms menu by pointing your mouse to the top or bottom right corner and select Settings. Afterwards click on Control Panel. Then navigate to User Accounts and Family Safety > User Accounts > Change Your Windows Password > Make Changes to my account in PC settings > Create a picture password. On the next screen, browse for a picture you want to use as background for the picture password. When you are asked to, perform three gestures on the picture you chose - you can do circles, dots and lines, where circles that are too small will be recognized as dots and any other form will be transformed into one of those three depending on where start and end-points are and how the lines were drawn.
The gestures you have drawn will shortly blink up afterwards, so you now if it is what you actually wanted to draw. After you have drawn three forms, you have to confirm by drawing them again. You can orient yourself on the forms of the picture but should not do so too closely, since it will be really easy to guess afterwards. In the following picture for example, one should not use the bubbles as line reference but look for other forms or links to use - make it easy rememberable for you but hard to guess for others.
If you forget what you drew or the system, for any reason, simply does not accept what you drew (which usually does not happen that easily) - there is still the option to switch back to the regular text string password on log-in screen.
If you want to make your Ubuntu installation a little less purple and orange, but don't see a solution that covers the change of log-in screen background picture change, try Simple Lightdm Manager. It is a tool that lets you turn the log-in screen into any image you have on your computer. To install, open a terminal and enter the following commands (adding repository, updating sources, installation):
sudo apt-add-repository ppa:claudiocn/slm
sudo apt-get update
sudo apt-get install simple-lightdm-manager
Afterwards open SLM and browse for an image you want to use:
Additionally you can alter the logo that is displayed on the log-in screen's bottom left corner.
Be aware though, that this does not work with images which are located in encrypted file systems!
While you needed third-party software on Windows XP to change nearly everything but your wallpaper, Windows 7 has made things quite a lot easier. To change your login screen, just create the following two folders:
Then open the Windows registry by typing regedit into a Run... prompt and browse the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background key on the left panel. Click it once and change the value of OEMBackground on the right panel to 1 (if the value is not already present, right-click the right panel and select New > DWORD-Value (32 bit)).
Afterwards choose a picture you want to have as your new login screen. It must be in the .jpg format and its size must be under 256 KB. Copy the picture into the created backgrounds folder and rename it to backgroundDefault.jpg.
Your login screen will now have changed upon your next log-off.