Laserfiche started out as an archival solution and encouraged the use of Black and White TIFF G4 format to reduce size.  The stamp needed to be easily created by the end user, but still needed to reduce size as much as possible, so the monochrome bitmap was used.

As the product developed, there were cases for color images being applied as annotations, so the Image annotation was added as an option to allow color images to be applied.  So while it is not a stamp, you do have the same functionality available through the annotations toolbar.