Using a Factory in Previews:
Add a factory to your image if it doesn't already have one.
Edit the factory's settings to the format you want to use.
Select the Window > Show Preview menu item to open the Preview window.
In the Preview window, set up the kind of preview you want and choose your factory from the Factory popup.
Now, your preview will reflect the options and format in your factory (for example JPEG images will not have transparent background).
You can change the factory used for the preview or select not to use a factory for previews in the Factory popup of the Preview window.