Well, you copy and paste, but you have to copy the location of the pic
You first copy the loction (URL- address), then you hit the IMG button on the post page, and paste the location in the box that pops up.
If the image is on your computer. you have to upload it to a server such as http://imageshack.us/ which will host your picture for free. It's easy to upload, and you'll be given the location to copy