It depends on where they're getting the pictures from and what they're doing with them. Some camera software will do it almost "automatically" if you set it up the right way. For example, "email picture" options usually will resize.
Another very popular tool is IrfanView, which can do a ton of stuff.
However, for a non-technical user (automatically implies that), I would recommend Picasa.
It's pretty, fast and has several good "1 click fix" options. If I'm not mistaken, they can go through their pictures, hit space to mark the pictures they want as favourites, and then select Export Favourites to any folder, at whatever size they want, with all their changes, comments, etc.
Matthias is right, it is wasteful, however I think that one or more of the galleries will resize automatically after you upload them.
If that doesn't work for you, there's a couple of other options.
Some of the other gallery programs, e.g. Coppermine or Menalto's Gallery,
will automatically resize when you upload. You can use the wrapper module to imbed a gallery installation into WB. The issues with doing that are how it fits in the template and it requires multiple sign-ins and procedures.
There is a Picassa option to export directly to your gallery but some of the forum comments indicate it's not the easiest to use.
http://codex.gallery2.org/Gallery2:Modules:picasahttp://missioncommunicate.com/blog/2005/04/27/picasa-coppermine-photo-gallery/ is a 5 step process which eventually uses the Windows Publish to the Web wizard.