You could look at a CMS… CMSMadeSimple tends to be my favorite. It probably isn’t good for large corporate sites that will need more advanced features, but it is easy to use, both for the designer of the site and for the client. He could use the gallery system of that CMS to easily add images.
Based on my experience, it sounds like Joomla may have more features and be more complicated to set up than necessary, and WordPress seems like it needs some hacks to be set up as a CMS, rather than as a blogging system. It is all personal preference… I am more familiar with CMSMadeSimple than Joomla/Wordpress. Perhaps if you know either of those systems thoroughly, it would be less difficult to set up.
Otherwise, you could also look at a gallery script of some kind, along the lines of Plogger (plogger.org) or zenphoto (zenphoto.org).
