Adding photos to existing entries in the database is really easy. Just go to the entry for the plant and scroll down to the bottom of the page. You'll see a link to "Upload a photo." Click on that and it will take you to a screen that will allow you to search for the photo on your computer.
Adding a new plant is a bit trickier, just because you need to know a bit more about the plant. If you click on the "Database" link on the left hand side of the page, that will take you to the main Database page, where you'll see a link to "Add a new plant to the database." Just follow the instructions. If you get stuck, a request for help can be made in the Plant Database Forum.
http://garden.org/forums/view/...
Hope this helps!