If you have a specific image that you want to use as your background, you can either:
Import the image that contains the foreground objects and then choose a template for the background. When that is in place, replace the template background with your preferred background image. For details, see Replace a background.
Create a template and replace the background with your preferred image. Then apply the template to an image or a batch of images.
For instructions, see:Apply a template to an individual image (free plan and above)