Welcome to the Apple Discussions. Terence is right. iWeb will do that automatically. Go into iWeb's preferences and check the box "Optimize images on import" for additional image downsizing.
For dial up there will be a lot of additional modifications you'll need to make in the site. For instance do not use photos for page backgrounds. Just use colors. Keep the number of items per page at a minimum. If you have a photo page double click next to one of the photos to bring up the Photo Grid pane and set the number of photos per page to a low value.
If you want to greatly increase the downsizing of photos you add to the photos page or any page, export them to a folder on the desktop. Next use
Resize! to resize them to 800 x 600 with a JPEG Quality level of 40 or lower (you can experiment with that to get the smallest file that would be acceptable to you). Use those new files in iWeb. If you go this route uncheck the checkbox I mentioned above. You don't want iWeb doing any other changes to the files.
For those photos that are not in a photo page resize them to the size you want before importing into iWeb. Once in iWeb go to the Inspector/Metrics pane and click on Original size. That will put them at the size you make them before importing and will result in a smaller size than if iWeb resizes them.
Also do not add any frames or enhancements to any of the photos you add to other pages. In the Photo page do not use a frame for the photos. Use only
Web Safe Fonts.
You can inquire in the iWeb forum for other options that you can use to make the site as small as possible. You will, however, lost a lot of the bells and whistles that iWeb offers in creating a web site.
If you're creating the site for only a very few with dialup you might consider creating two sites, one scaled down as we've discussed and one full blown for those with fast internet connections. You can put a link on the scaled down welcome page sending those with fast connections to the other site.
TIP:
For insurance against the iPhoto database corruption that many users have experienced I recommend making a backup copy of the Library6.iPhoto (iPhoto.Library for iPhoto 5 and earlier) database file and keep it current. If problems crop up where iPhoto suddenly can't see any photos or thinks there are no photos in the library, replacing the working Library6.iPhoto file with the backup will often get the library back. By keeping it current I mean backup after each import and/or any serious editing or work on books, slideshows, calendars, cards, etc. That insures that if a problem pops up and you do need to replace the database file, you'll retain all those efforts. It doesn't take long to make the backup and it's good insurance.
I've created an Automator workflow application (requires Tiger or later), iPhoto dB File Backup, that will copy the selected Library6.iPhoto file from your iPhoto Library folder to the Pictures folder, replacing any previous version of it. It's compatible with iPhoto 6 and 7 libraries and Tiger and Leopard. Just put the application in the Dock and click on it whenever you want to backup the dB file. iPhoto does not have to be closed to run the application, just idle. You can download it at Toad's Cellar. Be sure to read the Read Me pdf file.
Note: There's now an Automator backup application for iPhoto 5 that will work with Tiger or Leopard.
