how to tell upgrade vs built with SP2

I updated several XP images with SP2 before the Christmas break using SP2’s new sysprep and Ghost. Somewhere in the process I read that your not supposed to sysrep images that have been “upgraded” to SP2, that you are only to use images that have been built with an XP-sp2 cd, otherwise you have problems. Now, I dont remember which ones where built with sp2 and which were upgraded to SP2. Does anyone know A. how to tell the difference, and B. what problems that can cause?