Thanks Thomp for providing these screen shots to me.
Here is the basics of attaching a thumbnail to a thread
If you are starting a new thread, follow these intstuctions exactly.
If you are posting a reply, you must click on "

Reply." The following instructions will work from there.
(I know there are ways to do it using Quick Reply, but I'm trying to keep this simple).
1. Click on "Manage Attachments" under the "Additional Options" section. This will bring up another menu. If you have pop-up blocker enabled, you must allow this pop up (Click on the bar at the top of your screen and allow the popup)
2. Click on "Browse." This will allow you to look on your computer for the file.
3. Choose the photo you want to upload and click "Open".
4. It will bring you back to the pop-up menu. When you have all the photos you want to upload, click "Upload." You can close out of the pop-up menu.
5. You will be back at the Additional Options menu. The file you want to attach will be shown in Red.