Okay, before you can put it up hear, the picture must be online somewhere. For instance, if you want to post a picture of Chairman Mao, for instance, you could just go to google, find a picture of Mao on a site somewhere, and save the location by right clicking on it.Alternatively
If you have a picture of Mao on your hard drive that you want to use, you need to get it hosted. As I said, I use Image Shack, which you can find easily enough, but there are plenty of alternatives. Getting it hosted is just a matter of uploading it to the site, which you can do by following the insructions of whatever site you go to. When it's uploaded, it will offer you an image address. Finally
Now, take the address which you got from either hosting service, or the website (say it's called DancingMao.jpg, or something), and type it into your post like so:
That should do it.
here's a picture of Mao I just got (sadly, he isn't dancing).
Click edit on my post to see what I've done.