Adding a banner to your forum's header can greatly improve the amount of visits your clients get. This tutorial will show you how to place any banner or image inside your forum's header(fig 1)
fig1. 
Files to edit:
includes/page_header.php
themes/YOUR THEME/forums/overall_header.tpl (modules/Forums/templates/subSilver/overall_header.tpl - Default phpbb template)
Steps :
Open includes/page_header.php and find the following line of codes.
Above that line is where we will place our variable with the banner or image(text can also be placed here). Place the following codes there.
It is easy to edit, just replace the bold text with your link and image. Now that we have the banner inside a variable($wsads), we next need to output that variable to our template.
Scroll down a few lines and find the following.
Under that paste the following which will output the variable.
We have placed the variable $wsads inside WS_ADS which can be placed anywhere in our templates and it will out put the banner.
We are going to place our banner inside the header so it can be seen throughout the entire forums.
Open themes/YOUR THEME/forums/overall_header.tpl (modules/Forums/templates/subSilver/overall_header.tpl - Default phpbb template) and find the table at the bottom, it should look like the following.