This can done several ways. I usually do it from my class.
From class
.image
{
width:100%;
}
and for this your html would be:
<img class="image" src="images/image_name">
or if you want to style it using inline styling then you would just have:
<img style="width:100%; height:60px" id="image" src="images/image_name">
I however recommend doing it from your external style-sheet because as your project grows you will realize that the entire thing is easier managed with separate files for your html and your css.