You need to use the tags, and include a URL to the image you wish to include in your post between the [IMG] tags (i.e., where " and " is). In the example post you linked, they are using icon sprites hosted at http://pldh.net/, with the URL being formatted like so:
http://pldh.net/media/pokecons_action/X.gif
So you'd just replace X with the national Pokédex number of the Pokémon you wish to display.
For example:
[IMG]
The code looks like this:
[IMG]
For future reference, if you have questions like this, they belong in this thread. Hope that helps!