Background in web or app design can be an active real estate rather than a solid one color or pattern place holder.
Using background image can help connecting the site message to users better and faster. It also can create better emotional engagement.
Background image can be used in different ways:
1- One large image that fills the entire background
2- Repeating one image in x , y or both direction
3- Tiles of different photos
4- Blending one image to solid background
5- Using illustration instead of photo
6- Video or animation with or without interaction
7- Incorporating foreground and background for creating visual channel between them