Add HTML via TAG

Hi!

I wanted to know if it’s possible to add a tag when writting a new post or page, so it would call an HTML code.
Let me explain it:

I want to add, for example the next post:

“New World [yes]”.

Where [yes] = <img src=”../bla/bla/bla.jpg” />

So when the post appears, what we see is “New World + image”.
The way I think this would be possible is using the pluggin system: Making a new pluggin. But I just think it’s too much just to enter two lines of HTML code.

Thanks for replies!