Same issue for me too.
It may be related to b/c I am adding some custom size?
By in the setting I just use the “thumb”.
http://cl.ly/image/0W203W1v0Y1X
I found similar thread.
http://support.advancedcustomfields.com/forums/topic/thumbnail-preview-of-images-view/
But no luck.
Thank you for the help.
