The following is how to modify the file in iWeb for future slideshows.
1 - make a copy of the iWeb application as a precaution.
2 - Control-click on the iWeb application package and select “Show Package Contents” from the contextual menu.
3 - go to the iWeb/Contents/Resources/Scripts/Site folder and look for the file named iWebMediaGrid.js.
4 - for slide duration locate the following string and replace the value 5000 with the value you want. 1000 = 1 second.
5 - open the file with a text editor and replace the value 500 in these two text strings with the value you want. 1000 = 1 second.
6 - in this example I changed the slide duration from 5 seconds to 7 seconds and the transition time for 1/2 second to 2.5 seconds. With the transition I prefer, Dissolve, the longer transition time give it a smoother transition in my opinion. Also combined with the transition on each end of the slide the 7 seconds are reduce to about 5 to 5.5 seconds of full onscreen time for each slide.
NOTE: whenever there’s an update to iWeb you will have to repeat the modification to the application. So keep a copy of the edited iWebMediaGrid.js handy to add it to iWeb after any subsequent updates.xTo change the timing on slideshows already published and online just find the iWebMediaGrid.js file located in the Scripts folder for that website and edit it as described above.
To modify the site so it will work correctly after the MobileMe servers are discontinued on June 30, 2012, see Tutorial #26.
Thanks to Roddy McKay for his help in developing this tutorial.