can the picture or icon associated with the user at login be changed?

Yes you can change this in the Users & Groups system preferences. Go to the System Preferences (in the Apple menu) and click "Users & Groups." Then click the lock at the bottom of the window to authenticate, followed by clicking the picture in the account. In here you will have several options to choose supplied images, or take a picture of yourself with your iSight camera, and then crop the image and use it as the one to represent your account both at login and in programs like Mail.

These images should be located in the Macintosh HD > Library > User Pictures folder. Go to this location to see if the images are still there (they should be categorized into subfolders).

When the image is loaded, its data will be imported to your user account's directory entry so even if the image file is missing it will still show up.

If these images are available in this location, then try using Disk Utility to run a permissions fix routine on your boot drive (select it and click the Verify or Repair buttons in the First Aid tab), as sometimes minor changes to the permissions settings for these files and folders may prevent the system preferences and other programs from properly accessing them.

Have you tried to drag the desired picture to the Recents? Worked for me.

Is your problem solved or are you wanting to restore the default pictures? I couldn't tell from what you typed.

I agree there is a problem. You typed the originals are locked and read only. Looking at the folder Topher directed you to, select it and hit command I (Get Info). System should have read/write privileges. If not, unlock the box and set read/write.