You can add your own photos to your listing by first navigating to your dashboard:
Under downloads and websites, click on the listing that you would like to add your photos.
Scroll down to "Download & Arrange Media"
At the bottom of that section, there will be an area that says "Click here to add photos to this listing"
Select the photos you would like to add, and upload them.
Once they're done uploading, you'll be able to arrange them in the gallery.