Ok, so let's assume you know how to Make Placemarks and Overlays, and you know how to Post Placemarks. Then posting an overlay to the BBS will be a piece of cake. There are still a couple of decisions to make; Which Forum to post to, and what to name the Post.

Remember to make the name descriptive, as it will be seen by millions of users around the world.
It will be appearing in the "Google Earth Community Layers".

Three simple rules you MUST obey: (or no one else will see your work)

Save the Overlay Placemark to your local hard drive... Right click on the name in My Places and "Save As..."
Make sure the image you are using resides on the WEB or you are saving the file as a KMZ.
For images to appear in the description balloon in the Community Layer, they MUST reside on the WEB, not locally.

From here on out, posting an overlay is exactly like posting any other Placemark.

Remember, the Community BBS layers "i" are only updated about once per month.