why some thumbnails don’t load until refresh?

On one of my sites http://www.siiimple.com/ the thumbnail images don’t always load properly? When I refresh, sometimes they then load, sometimes they don’t. I have optimized these images with smush.it, but it doesn’t seem to make a difference. It seems almost random which thumbnails do and do not load.

Does anyone know a reason for this and perhaps point me in the direction of a solution?

Thanks
Justin