I have the same problem. It seems to be random. I uploaded 22 images this morning and two show as ?. It happened last Thursday as well. I uploaded lots of images, but two or three refused to show.
I also noticed that all the images in that gallery show a date of Jan. 1, 1970. The Jan. 1 date seems to be throughout as well. It’s not every image, but it is there at least once in every gallery I checked.
