It's not quite as easy to post here vs Facebook, but once you get the hang of it, you'll do fine.
When you're in Photobucket, move your mouse over the picture you want to post. You'll see a box come up underneath with 4 sections. The one you want is HTML Code. Left click on that box once, and it will be highlighted. Now right click on the highlighted area, select Copy. Come back to here, and in you message box right click again and select Paste.
You'll see a bunch of code pasted in, and that will become your pic. When you click Preview Message, you should see your pic!
Clear as mud? LOL