After ScreenSaver settings window is activated you can start configuring Mask. You will see 3 drop down menus and 3 checkboxes.
When "Cycle through all effects" checkbox is selected - you don't need "Transition effects" menu. In that case Screensaver will use all available effects to maximize slideshow experience.
If you wish you can uncheck "Cycle through all effects" checkbox and choose only one particular effect from "Transition effects" menu.
You can also change transition duration (1 - 10 seconds) and time for displaying each image (1 second - 10 minutes) in corresponding menus.
There is an option to show images in random or fixed order. You can change it by setting "Random Images Order" checkbox.
If you wish you may select the background color on which images will be displayed. This option will not take effect if image occupies full screen.
Setting "Rescale to fit screen" checkbox is very useful when picture size is smaller than your screen resolution.