Update May 19, 2013 - hey, they did add the word "Photos" in the latest version! Cool.
To add a photo to a Google+ post, click on the icon that looks like a camera.
It would be nice if they would add the word "photo", but I guess they felt like they didn't have the room. As a graphic designer and illustrator, I sympathize with their attempts to show a simplified drawing of something that looks like a camera. I'm looking at my digital camera right now, it's just a rectangle with a circle on one side and a square on the other. Lots of people take photos with their cell phones, so at a certain point making an icon of a camera will be just impossible. Of course, you could do an old-fashioned 35-millimeter camera, like you see the old-time reporters use in movies, but other than pros, very few people use that type any more.
And, just in case you're wondering, the icon that looks a bit like a cat's eye is for adding a video. The icon that looks like a car battery is for an event. And the sideways bow tie, which is actually supposed to be a chain link, allows you to add a hyperlink.
And no, there is no standard for icons. So if recognize an icon on your phone, it will be different on your camera, on the dashboard of your car, and on your computer. And, of course, once you recognize it, they will change it again.
I wish they wouldn't do this stuff, but they do. I'm sure they think it looks cool.