As part of OS X, Apple provides a number of styled screensaver options for displaying photographs and images you have stored in your iTunes or Aperture libraries when you are not at your computer. These options are fun to use; however, they are quite limited in the settings they offer, as there are practically no options to customize them.
At most, you can set the screensaver to shuffle the slide order and choose a source for the photos, but you cannot change the duration of the transitions or perhaps, more importantly for some people, the time in which the screensaver displays a specific photo before switching. The default value for this is 3 seconds, but some people may wish to have photos displayed for longer or shorter times than that.
Even though these settings are not available in Apple's system preferences, you can manually alter the settings files for the screensavers to adjust them.
The first thing you will need for this is a program that can edit property lists. While a plist editor is convenient, a quick solution is to use a text editor that supports authentication such as the free TextWrangler. This will allow you to edit system files without needing to change permissions.
With the text editor downloaded, launch it and then choose "Open" from the File menu. Then navigate to the following directory and open the file called "EffectDescriptions.plist":
System > Library > PrivateFrameworks > Slideshows.framework > Versions > A > Resources > Content
In this file, locate the key entry called "JustASlide" and then locate the child entry below this called "mainDuration." This file contains thousands of lines, so you can best locate these lines by performing a search for "JustASlide" and then scroll down about 20 lines to see the "mainDuration" entry.
You will see the default value of the "mainDuration" entry as 3, but can change it followed by saving the file. Since this file is owned by the system you will be asked to authenticate but it should save. After this, when the screensaver activates it will read the new value and dwell on a photo for the time you specified before changing.
You can always access this file again to adjust the value of this transition, and editing it in this manner will not harm your system; however, as with any settings or configuration change, be sure your system is backed up before you implement it.
This tip is a modification of the steps outlined here (thanks to MacFixIt reader Roger for writing in about this).