In principle you do not need to change the settings to use ImageGrab. You can use it directly. However, for advanced uses, you can access the settings by typing F10 on the keyboard or by clicking the Options button on the toolbar.
Note that you can also close the Options page at any time by retyping F10, or by clicking the Options button on the Toolbar. You can also click on the bottom right of the page:
In the Options page you will find 3 tabs: