Currently I use Surreal CMS, Pagelime CMS and LightCMS, all of these offer support for a lightbox. Surreal and pagelime have tutorials on using a lightbox in there systems so the end user can update the lightbox. LightCMS has the lightbox built in.
It looked good the Webassist CMS, but if it doesn't offer support or have some kind of image gallery included I'll keep on using what I'm using as the cost of the other CMS systems I am using is under half the price of Site Sculptor.