Did you use LANDesk's OS Deployment to lay down the image.
LANDesk lays down an image and places an LDISCAN.CFG file on the root of the C: drive.
One of the values in this file is the Last Deployed OS Image. When inventory runs, it should be populating that data. While this should be being picked up, it may not be because I think I remember a bug report that it wasn't being picked up, so that data may not be in inventory.
You could check to see if the LDISCAN.CFG is still on the c:\ drive. However, I think we had a customer complain that we were leaving files there so I think we are cleaning up those files now with our OSD tasks.
If you didn't use LANDesk to do OS Deployment, then knowing what image you laid down would be hard to determine unless you are marking your image in some way, such as a text file or a registry key or something.
I did a bug search and sure enough, there is a bug in our database:
CR00007461 - The inventory item "Last deployed OS image" isn't reporting properly.
It has only one request and so it lost priority and was marked as something we can try to fix in future and was not fixed.