I have a lot of clients that have image gallery’s, mostly done using ditto a couple of tv’s and lightbox or slimbox however these are set up from the beginning and when the client wants to add a image they just log in via the manager and create a document under the Gallery page, add the image through the tv and a title and click save and publish, this then displays on the front end.
There are also a fer gallery snippits on here that do the same sort of thing so maybe use one of them.
If the fact that a client may decide they want a gallery after the site has been built then I would guess that it is better for them to come back to you as it would be a chargable extra
If you want them to just have the abillity to install / use a gallery 3 months down the line and not come back to you then why not create one from the start and just not publish the page, it only takes 10 mins which you would save on the actuall build as Modx is easier to do than Drupal
Not sure if any of the gallery’s out there allow people to add from the front end but you could look into that.
