The Snapshot delay option (5) makes Spectacle wait a given amount of time (given in seconds) before taking the snapshot. This can be useful in many situations, for example when taking screenshots of drop-down menus
It's often useful to create a separate user for taking screenshots to ensure a standard look with the default settings.
Use Type: Color in the wallpaper settings to get a white background.
By default, pressing Shift + Print Screen will directly take a screenshot, without Spectacle popping out.