This is not really the place for HTML tutorials but it also depends what ethod you want to use.
if you create the whole banner (including the text) as an image it is just an image html tag like below
<img style=”display:block;” src=”ImageUrl” />
If you want actual text over the image, and if the text is the actual Title and Tagline used in WordPress, you could use the Xtreme title and tagline shortcode in a div positioned over the image. it would look something like
<img style=”display:block;” src=”ImageUrl” />
<div style=”position:absolute;top:0;”>[site_title matchtheme=true][site_tagline matchtheme=true]</div>