To add images to your listing, simply loging to your account and click the "Manage property and edit listings" button next to the property you want to add images to. From there, go ahead and click the "Edit Listing" button followed by the "Images" tab near the top of the page.
You'll then just want to click the "Upload" button and navigate to the images you wish to upload, and double left-click them.
You may also find a tutorial on adding, deleting and rotating images here:
https://www.youtube.com/watch?v=jbRXtHtngVc.