Each time I view a new gallery I recently uploaded, a few of the thumbnails fail to display. In their place is a small blue box with a question mark in it. When I click on this 'thumbnail', sometimes the full photo displays and sometimes I get another blue box with ?. It's never the same photos -- it moves around. I've tried re-uploading the affected photos and it doesn't help.
What is causing this, and more importantly, how do I get it to stop??