Kinda Long.
first. right above the post / reply box where You type in You message, click on the button that says "host an image".
then click "choose file". select Your pic and click open. then click "host it"
give it a minute.. then You have 3 options. the bottom one is the url. press copy. that will highlight it. right click on the highlighted url link. then click on the "image" button on the right side of the "host image " button and paste the URL.
For best results, before You click "host it", resize to 640px