First off, you'll need to be logged in and submit your photo into the gallery.
Then right click on the photo you want, and select "copy image address".

Then click into the thread on the message board you want, and click the "post reply" button. You'll see it at the top and bottom of the page.

This is where you can write a reply. To add your image, look for the little image square button and click on that.

A little box will pop up, and then you will paste your image link there. Either right click and select "paste", or use control + v on your keyboard. Click the ok button.

And there you go! Just click submit on the bottom and it will be posted to that thread.

Hope that helps!
