I am not aware of any way to query when you lasted imaged a machine from the console, within the console database using the builtin query tools.
I, being in the same situation you are in did something a bit different. When you do an inventory refresh, one of the things you get/can get is a software list. I added some information to the registry of machines via scripts to set the info I wanted.
I added some info custom info In the registry under:
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\*SomeAppName*
Specifically I had an application installed on the machines for copying the image files to the machine that I modified but you can do it any way you like. I added/modified two values to that registrykey. One was an entry called "Publisher" and the other was "DisplayVersion" Both are strings. Since you just want the date imaged you could add to either one the current date in some form of "after" script.
Now run your refresh inventory and that information will be stored in the database and you can query against the publisher or displayversion.
There are other similar ways to achievel this by "seeding" information that you can get the console to query in an inventory refresh you just need to figure out what you want to edit.
Currently though I just put the info in to the database directly via a service on my console machine because the inventory refresh bloated my database too much.