Adding your own User image in OS X
Convert the image to .png
Copy to the Desktop
Open Utilities
Open Terminal
In the terminal window type
cp /Users/osprdev/desktop/KeithRichards.png /Users/osprdev/library/containers/com.apple.ImageKit.RecentPictureService/data/ library/images/Recent\ Pictures
In this example I am using the filename KeithRichards.png
Then type cd /Users/osprdev/library/containers/com.apple.ImageKit.RecentPictureService/data/ library/images/Recent\ Pictures
You will need to edit the Recents.plist file to add your image.
So type pico Recents.plist
Look for the line
<string>811DF1F2-012D-4F49-826F-68034FCC716D.png</string>
replace 811DF1F2-012D-4F49-826F-68034FCC716D.png (or whatever is ion there) with your the filename of your image.
Save and exit the PICO editor
Go into System Preferences, Users and Groups, click on the current image, and choose the Recents option on the left. This will bring up your default image and one or more blank ones. Click on the blank one, click Edit, position your image and click OK.
Done.