Thumbnails when inserting images don't always show

Background: Recently upgraded to version 1.0.5 although the issue was occuring before upgrade.

When trying to insert an image some of the thumbnails show, some do not. Different images show/don’t show each time, there doesn’t seem to be much of a pattern. It doesn’t seem to matter how many images are in the folder. I’ve tried Chrome, Firefox and IE and it does it in all of them. If it was my site I’d just cope, but it isn’t so I need to fix it. I’ve attached a couple of images to demonstrate. Can anyone help?