You have to host your image online, imgur is a great place for that. Make an account, and then upload your picture.
Once you're there, click on your photo so its big, and then right click and select "copy link address" from he pulldown menu
click on the image file on our forum post, and it should make a ! [ ] ( ). paste into the parenthases. You can preview your blog post by hitting the "eye" icon.
Sometimes if you copy the image address before it's finished processing, it'll copy a link that won't work. When this happens the URL will start with "blob:". If this happens, just go back and copy it again once its finished thinking