If you are using GRT, which it sounds like you are, it does not follow the normal rules.
The IMG folders are created for each Backup to Disk that you do involving GRT. Those folders are only removed by backup exec when the drive/partition they are on is almost full. At that time, Backup Exec pops up an alert, and will automatically clear out any IMG folders that are no longer in the time frame set for overwrite protect.
Some people run a script to clear them out every so often, others manually delete them. I am giving Backup Exec another shot at cleaning them up after it didn't go quite as planned last time.