Once you access your "LISTINGS" under "MANAGE LISTINGS" in the top menu, you can upload a listings gallery per listing and upload your gallery of photos.
Click on the yellow "Photos" button.
Here you can Select or Drag and Drop photos.
For the main photo of your listing, hit the yellow "Attachments" button instead of "Photos" and you can Select or Drag and Drop as well.
Make sure to hit "Crop to Selection" afterwards.