All of the other items are empty boxes.
This also is the case in the Asset chooser dialog.

Even the first box can be empty.
Using developer tools we can see that it successfully grabs all ofthe images - so it is simply not displyaing them properly.

When one clicks on the item it shows up in the preview.
Once it has shown up in the preview - then sometimes it is rendered in the grid.

In the grid:
/magnolia-bundled-webapp/.imaging/thumbnail/contacts/6fdf160d-0dee-4419-959e-ce805c9fe5b3/schopenhauer2.2012-08-24-17-28-09.png?cb=1371466515791

In the preview:
/magnolia-bundled-webapp/.imaging/portrait/contacts/6fdf160d-0dee-4419-959e-ce805c9fe5b3/schopenhauer2.2012-08-24-17-28-09.png