Go to the Multi-Plant photos link in the "Goodies" and click on "add an image." Then you can go in and put tags on the individual plants in your photo. Sometimes it can be a bit tricky, especially if you're a fumble-fingers like me!
It will give you the option to draw a little box around the plants in your picture and add a description. It's generally easier to open a separate tab in your browser and find your plant in the database; then you can copy the plant ID number (it will be in the blue column on the left) and just paste that into the description, instead of trying to type the name correctly. Heh, at least I *think* that works. Haven't done it in a while.