Thumbnails blank on my Gallery page.

I’m a total novice to WordPress but I’ve figured out how to add photos to the Gallery. The problem is that, while most of the uploaded photos display as thumbnails on my Photos web page, several of the thumbnails are blank, even though the photos were properly uploaded and show on the Gallery.

For now, I’ve deleted the photos that are showing up as blank thumbnails, but I’d really like to have them back and showing up.

Can anyone guide me in the right direction to sort this out?

Thanks much,
David Cooper