How are the photos from the plants database that post to the homepage selected? Are they drawn from users plant lists? If so, can this selection be blocked by the user? Can plant photos be submitted for selection?

They are the latest additions to the database.

You can customize your home page to not see those photos.

Yes, to customize your home page and have it appear to your taste scroll all the way to the bottom of your home page and you will see the link.

Or you can click on your profile, upper right, where you can select the Settings from the drop down menu and edit your preferences and adjust your homepage.