Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
If you look along the toolbar at the top of the posting box, you'll see a picture next to the world + chain icon (hyperlink) -- this is the Insert Image icon. Select that icon and input the link to you picture, hit 'Ok' and there you have it!