v I believe all you have to do is upload the photo to a sharing site like Photobucket or Tinypic and post the direct link into that box. It's been a long while since I posted an article, however I do believe you just need to post the direct link in the box and the admins/article mods will take care of the rest.
I also forgot to mention that the image will also appear in the article itself. For example, click on the 'Articles' section and then click on any of the articles. You'll notice a small picture next to its title and then a larger one in the actual article.
The intro image is the image that will appear if your article makes it to the homepage. Like if you go to the Mibba homepage, you'll notice that the top articles of each section have an image next to them. That's the intro image I do believe. I hope that made sense. ^^'
The 'lead' is basically just a small description of what the article will be about.
I also forgot to mention that the image will also appear in the article itself. For example, click on the 'Articles' section and then click on any of the articles. You'll notice a small picture next to its title and then a larger one in the actual article.