Background images are gone in Blender 2.8, at least in the traditional pre-2.7# sense. The old system was aging and prone to failure … and it has been retired due to inherent limitations.

Information was stored as User Interface data, as such if you opened a file without the Load UI option ticked they would appear to be missing, and any previously assigned background images mysteriously absent.

Saving the file would then silently erase them, permanently eliminating without warning or explicit user intervention.

As an alternative to Gwenn’s answer, you can also use an empty. Empties in Blender now have an image type.

This is very useful as it gives you more control over positioning your references and users coming from other software can use this method as it is the norm for them. (most Max users I know use frozen planes with textures when modeling).

Control the image object properties in the “Object Data” tab:

Where to edit image data in Blender’s maze interface

Once added, you have a lot of control over the reference. You can adjust the opacity, scale it, rotate it etc. Much more flexible than the common way using the N panel in the other answer. You can also lock it so it behaves the same way. Another advantage this has is that it’s visible in both orthographic and perspective views.