What's it mean about one in 100 images have red X in white box?

Answer 1

It means the link is dead or broken.

Sometimes that's a result of a time-out (you can hit refresh and see if it comes up). You can also click on it because sometimes the thumbnail link is broken but the picture you're linking to is still okay.

Answer 2

It means that the files are either currupt, or if you are viewing them online, then they may have been removed on teh other end.

Try refreshing the page