I believe there are two ways to insert a pic in your post. One is to download (upload?) to this site using the "manage attachments" button, and the other is to link someplace off of this forum. I suspect this is the difference.
Edited to add: This thread has both. Notice the pic I added has a frame with "attached images" around it, but the pic in the original post doesn't. Only one shows up when you hit post reply.