You will need to have an FTP client and access to your server. If you have cpanel for your site, then the FTP details are normally your URL with your cpanel login details. Different setups have different ways of working. A good (cheap, and has a 30 day free trial) FTP client is
FlashFXP.
Once you have logged into your FTP then you need to locate your forums on your server. Most styles require the images folder to be uploaded inside the images directory, but check the instructions that come with your style, or with the style creator if you are not sure.