If you want to upload a new or different main image, the process is simple.
Go to your listings via "Listings" tab at the top of the page in your backend access to your new website, then click the "Listings".
Click on the "Lead Photos" button on the listing you want to add the main picture. This will take you to a new page.
Select or drag the picture you want to the "Main Photo Detail" or the "Main Photo Listing Gallery" squares
Select the picture and click the yellow tag "Crop to Selection". Wait until your picture is done cropping, then you are done!