If you go to
http://elementswiki.co.cc/ , you can see that I added some "main" images to some of the articles, and I'll add more soon.
The plan is that each article will have one "main" image. Advantage of this is that the cool thumbnail image you see there will be created automatically and our wiki looks much more professional.
Basically what you need to do is this:
1. Upload an image to Wiki (yes, it must be uploaded to Wiki. Don't use a hosting service like Imageshack)
2. Copy image url
3. Paste image url in the "Post Image" text-field
Done.
Your image will appear
after the title and before the content. Thumbnail will created automatically.
I'll do an advanced tutorial later.