Adding a Background Image in Blender 2.8+ is gone, at least the way it worked in previous versions as a component of View » Properties. Instead an editable background plain is now used to provide similar capabilities.
Design note: a background image can be added to a scene using several different approaches in Blender 2.8+, whereas for Blender 2.79 and below they are loaded as a component of the 3D/2D grid that would only display in Orthographic orientations (Front, Right, Top etc.). Using an independent object works around this limitation.
For Blender 2.8+ a scene can be assigned what is effectively a background image using a dedicated object type called Image. Two variations are provided, Reference and Background, the difference largely being where the object appears – in between other objects in the scene (Reference) or behind them (Background).
Design note: the Image object being an editable entity potentially means it can be used for other things besides being a repository for a background image, e.g. animated components of a scene and so on.
To add either, in the 3D View click the Add menu (Shift + A), select Image and then either Reference or Background as required – Add » Image » Reference/Background. This opens the File Browser. Browse to and select the asset to be used then click the Load Background Image button upper-right. Blender will switch back to the 3D View with the selected image displayed in an image plain orientated face-on to the viewport camera ready to use.
Design note: the 3D Cursors location, and the general orientation of the view, are used to determine the Image objects initial orientation and placement in the scene so may not appear where needed. To compensate or correct for this, select the plain, access Object Properties and adjust the Location and Rotation values (below first). Alternatively use the transform manipulators, Move, Scale and Rotate to modify or adjust the plain, or mouse over a edge or corner to reveal the inbuilt manipulation controls – click-drag and modify (when using the Transform widgets adjust is made relative to the objects Origin, using the inbuilt controls the plain adjusts proportionally).
The Image object can be modified using the standard manipulation widgets – Scale, Rotate and Move – or using the in-build edge and corner control elements (shown above highlighted yellow) which change the plain/surface proportionately depending on the action used.
Adding a Background Image object to a scene, a dedicated object type that can be freely manipulated as needed once an image is loaded in for use – the plains dimensions automatically adjust to the image being loaded, shaped to fit its (texture) bounding box (the overall height and width of the image).
Similar to the Image object, an Empty can also be used. Here a dedicated type called Image is available. To place an Image Empty to the 3D View, click the Add menu (Shift + A), Empty, then select Image – Add » Empty » Image. A wireframed object will appear at the 3D Cursor. Ensure this new object is selected, click Object Data properties (green axes icon) and then the Open button in Image options. In the File Browser find and select the image to use as a background and then click Open Image top-right to load the selection into the Image Empty ready for use.
Design note: for all intents and purposes the Image Empty and Image Object differ only with respect to the underlying datablocks that define them, one being an entity the other being an object; both are subject to the same Object property adjustments (Rotation, Location, Scale) and Object Data settings (Depth, Size etc.) – care should be taken not to confuse Object properties with Object Data properties, and vice versa, they do differ.
Once the Image Empty is available, in Object Data properties its behaviour can be modified to suit requirements, for example changing the Depth order it appears in the scene relative to other items and so on.
Brief overview of the two main ways to add a background image to a scene, as a Background object, then Image entity.
Loading a Background Image into Blender 2.8+