There seem to be 2 ways to add reference images in Blender as shown here https://youtu.be/PTxWKZZbIvU
Firstly you can hit Shift+a>Image>Reference, and secondly you can select a camera and in the Object Data Properties section there is "Background Images" which does the same thing. What are the pros and cons of each approach?
Also, How do you add a background image to the orthographic views?
To view through the selected camera press numpad 0.
To align the Reference image with the camera view, select the Reference image, then in the header menu > Object > Transform > Align to Transform Orientation then in the Operator box choose Orientation > View.
Reference and Background are empties, they won't be rendered, they are used as references to model an object, liked the former Background in 2.79.
Actually they are exactly the same objects, except that the default settings in the Properties panel > Object Data are not the same, by default Background will only be seen in orthographic view and it is set to have its back transparent (but you can switch off these 2 options or switch them on for a Reference image).
What's the difference? I guess it all depends on what settings make you comfortable to work with. The Background version works a bit more like the former Background in 2.79, it makes the image more discrete and transparent in your scene, and if you enable its Only Axis Aligned you'll see it only in perpendicular views, which is very convenient.
Reference and Background are just two presets for an image object :)
The settings can be changed at any time through the Properties panel.
