Each physically discovered file should have a discovered date that would allow you to see when it was first detected, but there isn't a specific report that tells you what was installed after a machine was imaged.
LANDesk Silver ESP
The One-Stop Shop for LANDesk Enhancements
I think you will need to have a report of software installed on a freshly imaged machine. Then when you report on software installed on other machines you can pull it into excel and compare the two sets of software for changes. I am tasked with the same thing now, but have just started looking into it. Please post if you have found another solution.