Hi Dave,
Yes, this will make all of the forum feature instances have the same logo. To give each instance a different logo, use the "Custom CSS Class" option in the Instance Settings and then use that custom css class in place of the .forums class. So, it would look like this for a forum instance using the MyVeryCoolForum custom CSS class.
.MyVeryCoolForum h2.moduletitle{background: transparent url('nameofimage.png') no-repeat left center; padding-left: 20px;}
HTH,
Joe D.