all images gone after update, but show when logged in?

I’ve tried the usual troubleshooting 101 steps, but it still doesn’t make any sense to me. Figure I should ask for help before I screw something up.

THE PROBLEM
All images are replaced with plan, white boxes. Size, shape, placement are correct, but just white boxes. They will open in a new tab, but all come back ‘placeholder.gif.’

WHAT MAKES THIS STRANGE (TO ME)
As soon as I log in, the images all re-appear! Then they stay there all day long. (Suspect cache, but seems like they’re still there for me even after emptying the cache.)

I’VE TRIED THE USUAL
– disabled all plugins, deleted cache
– re-activated plugins one-at-a-time, deleting cache in between
– confirmed 755 on my uploads folder

ALSO TRIED:
– rolling back NextGen Gallery 2.0.7 (looks like they’re having issues)
– jumping into NextGen Gallery beta 2.0.9

At first, everything seemed to work until I re-activated Yoast WP SEO. Seems like I run into compatibility issues with this plugin a couple times a year. No biggie.

Hit the site again tonight, though, and the problem is back – despite WP SEO being deactivated. Uh oh. Went to update/delete my Yoast support thread and it looks like it was already deleted. (Thinking I don’t really care about SEO anyway.)

I’m using a minimally modified (as in merely spacing between titles, tags, content, maybe a font change) child of the super lightweight “Frank” theme.

All images work fine when I switch back to 2013, but they also work when I jump into the default “Frank” theme. AND they work once I’m logged in.

I gotta think there’s something very, very simple here. If the theme was incompatible, the images would never show up. It’s got something to do with my, personally being logged in as far as I can tell.

Any ideas?