You're pretty close to getting the hang of it. Like I said, after you upload your picture, just copy the stuff in the box under "IMG Code for Forums & Message Boards" and paste that in your response.
You should be able to just click the box with your
left mouse button, it will highlight the junk in the box, then click the
right mouse button and a small window will pop up with "copy" being one of the options. Roll your cursor down to copy and
right click your mouse. Come back to your window where you type your response and paste what you just copied. Hit preview. Your picture should show up. If it did, hit "post" or whatever it says and you're done.