How do I change the FileVault login icon on OSX Lion?

Since the hard drive is encrypted, the login screen cannot show these files directly before you unlock the drive by logging in. If you activate File Vault, certain resources, like the Apple icon, are placed on the Recovery HD partition for presenting them.

These resources are recreated every time you change a setting affecting the boot login screen, e.g.

• System Preferences » Users & Groups » Login Options » Display login window as…
• System Preferences » Security & Privacy » Disable restarting to Safari when screen is logged (i.e. "Safari guest account")

Change the files you know already to the desired resource, and toggle one of these options to recreate the resources on your boot disk.

You can test correct presentation by activating Fast user switching menu in Login Options of Users & Groups preference pane, and selecting Login Window… from the new menu item.

Alternatively, you can edit the /System/Library/Caches/com.apple.corestorage/EFILoginLocalizations/loginui.efires file in the Recovery HD partition's rock-paper-scissors com.apple.boot.X folder manually using the information in this topic. The Apple icon is the entry named loginui_apple.png.

Be aware that your changes will be overridden whenever you change a relevant setting in System Preferences.

BootXChanger might be what you need. It has instructions on the download page.

BootXChanger will allow you to change the boot image and background colour on Mac OS X 10.4, 10.5 and 10.6

To set an image as a boot image, open BootXChanger and drag it to the image well, and click Apply. You can also click the background colour to change it. You will then be asked for an administrator password, as the boot image is stored in a system file.