I have found the phpBB.com web site very helpful on giving step by step instructions to customize the forum. Their
Knowledge Base page is very easy to follow. To change the logo, go to the Styles section on that page. You will find many other instructions to change the look of the forum. If you use Dreamweaver, there are directions to give it the ability to edit the templates (.tpl files) that the forum uses. If you need to know what all the templates do, check out
this list, it explains what each is used for. Be aware that the templates are not html files.
Good luck on your customization.
Ray