Use img tags like this: .
For example here's the URL of a Bulbapedia image of Sharpedo:
http://archives.bulbagarden.net/media/upload/thumb/a/a8/319Sharpedo.png/180px-319Sharpedo.png
Putting it in img tags, you get:
[IMG]
Alternatively, you can copy the url of the image you want, click this button: [IMG] and it will automatically format the image.
If you're using Chrome, use Preview Message/post to see how the image will turn out. (Chrome isn't the best browser for Smogon sometimes.)