1. How to add a background to VIEWPORT only for work reference:
go to (Menu) > Views > Viewport Background > Show Background (check) > Viewport Background (Or just press Alt+B)
click File (Viewport Background window)
*This image will not render.
2. How to add a background to RENDER:
go to (Menu) > Rendering > Environment (Or just press 8)
Choose a solid color (click Color) or insert an image (select Use Map and then click Environment Map)
for a solid background, pick a color:
for an image, click Bitmap and choose one.
* Will render this time.
3. Want both? Do the both.
