How do I put a backdrop behind the model that I am creating so that I can compare the picture to the model while editing?
See Cannot find Background Images feature in Blender 2.8 if using 2.8.
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:
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.
In the View Properties panel (accessed by pressing N), there should be a category called Background Image. To enable it check on the box to its left.
To load an image, press the 'Add Image' button. All of your relevant image and display options are included in that menu.
However, remember that background images will only be displayed in:
Camera Perspective view (Numpad 0)
Any of the preset orthographic views which you can access using the keys on the number pad:
- Front/Back (Numpad 1 or CtrlNumpad 1)
- Right /Left (Numpad 3 or CtrlNumpad 3)
- Top/Bottom (Numpad 7 or CtrlNumpad 7)
Background images will not show in perspective view. To switch between perspective and orthographic view use Numpad 5
Background images are used for reference only and will not be part of the final render.
Starting with Blender 2.8, you can do Add -> Image --> Background
When I am trying to adjust a camera's transform to match an existing image, I add the image as a background for the camera, not the viewport. The "Image > Background" answer by @MCCCS adds an object into the scene with a specific rotation that matches the current camera; when you adjust the camera the image no longer fills the viewport.
The simplest way is to drag an image from the file system onto the viewport when your camera is active; this will add a Background Image to the active camera.
Alternatively, select the camera, and in the Properties panel select the Object Data Properties tab (green camera icon near the bottom), scroll down to the Background Images section and click on Add Image.
The image remains unchanged on screen as you transform the camera until you can get the grid or objects lined up with your reference content.