Open a NEW TAB to the gallery, go to your picture. Now, down the right hand side of the page you should see a column of smaller, thumbnail-sized images.
Right Click, choose "Copy Image Location"
Now come back over the forum thread, and click the the Insert Image icon (it looks like a picture of a tree) and paste into the blank spot, hit OKAY.
What you should see is the tiny image of your page. Now you want to link that image back to the gallery. So, click the image (not right click, just click). It might turn blue, you might see the bounding box show up around it.
Go BACK to the gallery, copy the URL from the address bar at the top, and come BACK to the forum thread.
With your picture highlighted, click the Insert Link icon (it looks like the world) and paste into the blank spot, hit OKAY.
Again, you might see the image turn blue or something. That's fine.
When you post your reply you should have a linked thumbnail image that we can click on to go see your page.