Don't be too sure it is anything you have done wrong at your end.
I was working up a post with five photos. I went to adjust the text toward the end of the post, hit preview to see my edits and the system had taken the first two photos and sent them "Invalid Link" and left the other three photos intact.
I dumped the whole thing and started over.
I ran some tests on uploading photos and could not find anything that triggers the "Invalid link" thing. It appears to be almost a random event.