Once you upload a photo, you should actually need to go to the plant article you want to add it to, click on the "edit" tab, and in the section that is called Gallery, you just add a new line that starts with either "Image:" or "File:" and add the name of your file after it. You'll see how the other photos are done when you are adding yours, and as always with a wiki, don't worry about making mistakes, and always feel free to ask questions :-)
Thanks for adding the Cordyline australis var. 'Red Sensation' page, Stoneangel. I added your photo to the page as well, hope you like it!