It will open up a box where you can upload a photo from your computer. Mouse over the blank face and you will see a message.
From there just follow the steps to add a photo from your computer. Keep in mind that the photo will have to display fairly small, and in a circle. Also, please resist the temptation to use a photo of your cat, or Iron Man, or what you looked like in kindergarten. Google+ is a place for grownups. If you're not comfortable using your real face and your real name, Google+ is not for you, nor is any gathering of intelligent adults. If you must be "Miss Cat Lady" please wander over some anonymous place where you can post with a mask on.
Google+ is a great place, filled with intelligent and interesting people. And you have a great face! Let's see it!