Easy enough. Start by opening an account to host the pictures, if you don't have one already. I use photobucket, it's easy to use and free. The only drawback I have found so far is that it resizes all large images down to make them 250kb.
Once you have the image uploaded, it will give you a url for that image. Use the image button (looks like a mountain scene) above, or wrap the url for the picture in tags. And you're done!