Easiest way and this work with all styles.
go to your forum, on the logo right click over the logo, click properties make a note of the route to the image, in this case /testforum/styles/prosilver/imageset/site_logo.gif
Now open your ftp manager and connect. follow the route you got in the properties, ie: /testforum/styles/prosilver/imageset/site_logo.gif copy the image down to your cp . now check size and file type.
Make your own image, name and save it in the same name and file type as the one you download.
then all you need to do is upload the new image back into the same place.
Job Done.
this works with all styles you install once you become acustom to changing you can start making complected changes, as seen on mine
Here