OK, in my case, I would have to remove all plugins and put them back one by one until problem resolved.
$cd mywebsite/wp-content
$mkdir a1
$cd plugins
$mv * ../a1
$cp -R ../a1/<one plugin at a time> .
Apparently, the plugins exhausted server’s operating memory and that affected image loading process, at the end, go with the advise from John-Paul, and remove unnecessary plugins, specifically boldgrid inspiration..etc, anything you don’t need after initial start up.
Good luck!