well i dont know what you mean by phpbb2, but i'm assuming phpbb2.0.x? some people think phpbb3 is phpbb 2.1 =/ soo...
if indeed it is phpbb2.0.x then you'll find that under [base dir of phpbb] > [templates] > [template name (default: subsilver)] there should be a file called overall_header.tpl the only difference between a .tpl and a .html are that the .tpl is just an extension used to tell you its a template file, inside it is just plain HTML with extra tags like {something} that the php parses when it takes the template to display.
so, if you know some basic html you should be able to handle it from there.