Mike -- welcome to 78TA.com!
What you want to do is to go to the picture in the gallery and get the "IMG" link. Copy that entire link -- "IMG" tags and all -- then paste it into the post window here on the forum. Use the "Preview" icon to check it to see that it comes out, and when you're happy with it just hit "Post" as always!
Raise your hand if you have any problems!