I experienced this just recently as well, when I FTM-uploaded a small segment of a private tree in response to an inquiry from someone so that they could view it.
The "grey" images appear to have been temporary thumbnail placeholders which have, by now, been converted to normal thumbnail images.
Perhaps just a glitch in that thumbnail processing that has been, by now, rectified? Perhaps the OP might consider revising this topic title to reflect that? The images being uploaded are normal, not "grey".