Adding large textures such as backgrounds to the atlas isn't a good idea; you'll waste space on your atlas and won't gain performance since backgrounds aren't usually used very often.
A simple way to display large textures is to use the UITexture component.
Let's use the UITexture component to display a background for our menu:
Select our
UI RootGameObject.Hit Alt + Shift + N to create a new empty child GameObject.
Select
UI Root|GameObject., which is empty and new.Rename it to
Background.
Click the Add Component button in the Inspector view.
Type
texturewith your keyboard to search for components.Select NGUI Texture and hit Enter or click on it with your mouse.
We've attached the UITexture component to our Background GameObject. We can now review its parameters before we configure it.