The pic that is not showing is in one of my albums that is set to public and assigned to a category that allows for viewing to unregistered users.
These checked usergroups can view images in this category.
Unregistered / Not Logged In = YES
That same pic (although smaller) shows in the latest comments box just below it, why wouldn't it show in the featured box / most popular?
I have "Require users to be registered to view images" set to yes, but that shouldn't affect anything as all the other thumbs show everywhere else. I use that for when they click on a thumb they get the "you must be registered to view images" message.
Maybe an oversight? Would be nice to have blank spaces purposely being created. Kind of wastes space and looks, well not so good. Maybe something that can be revised in the next release?