MacOS – Password dialog box differs between Macs

I upgraded a Mac in my lab (23" iMac) to Lion. My MacBook Pro (mid 2007) is running Mountain Lion. For some reason the iMac is showing a different user password screen than my MBP – I can't figure out why. I like the new screen, and am trying to figure out why my MBP doesn't use it.

My MBP shows the "old" screen, with text boxes for user name and password, like this:

This screen hasn't changed on my MBP since Tiger at least.

The iMac shows only the user icon and a spot for password, like this:

I have a hard time thinking it's a hardware limitation issue. Could also be from some old setting, but the iMac used to use the old dialog too. (iMac originally ran Leopard, was upgraded to SL some time ago; same basic upgrade path as my MBP). Any ideas?

Update: This is still happening in Mavericks. The single answer below does not address the issue – the preference pane affects the login screen only, not the password dialog that comes up after screen lock. (I'd vote the answer down – not because it's wrong, but because it doesn't directly address the problem – but I apparently can't do that yet.) User icons on login look just as expected – round icon with user photo, but the password dialog after screensaver has not changed on my system since I bought it in 2007.

Per my comments below, note that any new accounts created use the new screen lock dialog, but my account uses the old format – so this MUST be a per-user issue. I am having a hard time accepting that no one out there can tell me what needs to be changed. I've tried nuking basically ever preference I can find on my system. This is really minor but trying to figure out why it is even happening is driving me nuts.

For anyone else with this issue (and at least one person has emailed me to report they deal with this too!) – I did "fix" the problem thanks to a hard drive crash. I did a clean install, and did not copy any Apple system prefs from backup, just my files and program preferences. This resolved the problem, but didn't do so in a way that allowed me to figure out what the specific setting was that caused it in the first place!