Depends on your browser, but why don't you right-click over the image and click "View Image", (Your address bar will then reveal it's location) or look in the source code of your forum and you'll find the simple forum folder is at:
/images/forum.png
(Obviously there are other images for pinned folders, etc.)
So now all you need to do is search through the NAML macros to see where that link is found and replace it with a link to your preferred image.
If you post a copy of your preferred image to your forum it will even be stored on the Nabble servers, so it will load faster (Don't ever delete the post that includes the image or Nabble will delete the attachment with it!)
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.