Hi Experts

I have created my HTML website. It has 20+ pages. Now I want to place a banner on top of all pages. One option is to place that banner in every page one by one.

I know there is some other way by which I will be able to put the banner somewhere only once and then it will appear on all pages. I should have created a template first before starting to create html pages... but what can i do now. how to avoid putting the banner in every page, and put it in some place once so that it will appear in all pages where i want it?