When you change the Picture for the Guest account, via User Accounts applet, the original guest.bmp (with the briefcase icon) gets replaced.

If you with to revert back to the same old icon for your Guest account, download Guest.bmp and then place the file in the following folder:

%allusersprofile%\Application Data\Microsoft\User Account Pictures

Type the above in Start-Run dialog. Then place the downloaded guest.bmp file in that location. Overwrite if prompted. Note that the following folder holds all the default picture files, except guest.bmp.

%allusersprofile%\Application Data\Microsoft\User Account Pictures\Default Pictures