Wait -- I think I guessed it. You said "I use the "HTTP://" button" -- I bet you mean the "IMG" button near the top of the posting window, right? That's for linking to a pic that's already on a web server somewhere. When you want to upload a pic directly from your hard drive, click on the big "Browse..." button under the box you write your post in. On the left it says;
Attach file:
Maximum size: 102400 bytes
...and the "Browse..." button is in the middle of the screen.