If you upload an attachment, and the original is larger than the presentation, it usually expands. However if you drag-n-drop a picture into the post, it will not expand. Here's three samples, the first is a direct link off the Internet, the next is an upload attachment, and the final is a drag-n-drop into the message body.
Here's a 1680 x 1050 photo, I'll first use a direct link from the Internet site.
Here it is using the Add Attachment feature. Notice this one expands when you click on it.
Finally, here it is by dragging and dropping it directly into the post. Notice this one doesn't expand when you click on it.
