Images missing until cache cleared

A recent (in the last month) problem has me flummoxed! The symptom is that after a day or so of normality the site fails to display images. The image place holder has just a question mark. Usually it’s the same images that don’t display but not always. I can get the site back to normal by clearing cache but after a few days the same problem reappears. I note that more pages than actually exist are logged as being cached. Any suggestions what might be causing this problem?

http://ww.wp.xz.cn/extend/plugins/wp-super-cache/