I use photobucket, very simple to use. You upload your image(s) to it, go into the album you've stored them in then you have 4 lines of code underneath each one. As has already been said you need to click the 4th ( lowest ) line which is titled "IMG Code" then just paste it into your post.
I tried a couple of others (imageshack and flickr) but photobucket works the best for me, I just found it the easiest to use. I'm sure there will be conflicting stories but for me, it's the best
