You can upload and add your own photos or replace existing images in the design if the template designer has enabled this feature for your design.
If you are editing on a mobile device then you will only have the option to replace existing images-- not upload any additional images. Uploading additional images is only available on a computer/laptop.
To add your own images into your design:
Select the photo you would like to replace and click " replace image" in the panel on the right.
To upload your own additional images select " Add New Image" on the panel on the right. If you cannot see this option then the seller has not enabled this feature for the design and you will need to contact them to request this button to be added to your template.