Hi Sarah --
where do you want the photo to go? into a forum post, or the plant database, or ?
When you make a forum post you should see a button for "upload image" -- just click on that and then select a photo from a file on your computer or wherever. (after uploading you can click on "preview" to make sure you have the photo you actually wanted to add)
For the plant database, go to the plant entry and scroll down a bit to "adding actions" and "upload a photo."
Hope this is helpful -- if not, let us know and someone will certainly be able to help out!