First of all, the ones you say are symbols, not pictures, do you mean the PDFs? Because while you'll only see the PDF symbol, if you click on it you can open the PDF.
If you go to the photo page, you should see a section called People, Places & Things in the Photo. Under that you'll see the people in the photo listed, and each one has the following options: uncertain, certain, and REMOVE from photo. Select the last one and click the Save Changes to Photo Info button further down the screen. That will remove the person from the photo. When a photo has no people attached to it, it will eventually be deleted.
To select the photo you want to appear on the tree (the primary photo), look for the Set to Primary Photo section on the image page.
I hope that helps!