To add an image as a background follow the steps below:
If there's a dark overlay, select it and click Arrange > Back so that you can add your image to the bounding box underneath.
Upload your image by clicking Upload your own images or simply dragging your file in.
Drag your image onto the placeholder box. It will automatically crop to fit that area.
You can adjust the image within the box by double clicking on it, making your edits, and then hitting return on your keyboard.
If you need to move your image behind a dark overlay or other graphics, simply click Arrange in the top right corner and click Back or use the Control/Command + [ shortcut.
For more help using Canva, head over to Canva Support where you can search their library of support articles and even get in touch with them directly.