Modifying Template

Usually when you want to write a new post in WordPress you press ‘Write’ and you’ve got a blank sheet both in HTML and Media.

Now, I would like to add this piece of coding (a definition list) to show by default when you want to write a post:

<dl><dt></dt><dd></dd></dl>

I will add my picture between the <dt> tags and the text between the dd> tags.

I don’t know whether you have to modify the template or not but how do you manage to do this?